I think most hobbyists would be surprised to learn how much of the information that's so often repeated as truth, even in some of the most popular books in the hobby, either goes against established research or has never been demonstrated. These tend to be a lot easier for a marine biologist like myself to spot than for most people, so I figured I would start a list. If you can think of any, by all means add them. I just ask that you play by one rule so that we don't create new myths of our own- Please limit responses to what is known empirically (i.e. via controlled study) rather than simply from anecdote and personal experience, what you've heard from other hobbyists, or what you read in a hobby publication.

A good place to start is with Eric Borneman's series on the subject.
http://www.reefkeeping.com/issues/2003-11/eb/index.php
http://www.reefkeeping.com/issues/2003-12/eb/index.php
http://www.reefkeeping.com/issues/2004-01/eb/index.php

Some of my favorite myths:

Crustaceans need iodine for proper health and molting.
This one probably comes from the fact that shellfish is often said to be high in iodine and that adding iodine to a tank can induce molting in shrimp. Well, crustaceans (and many other animals) do need a very small amount of iodine for health, however this is obtained from the diet, not from the water. There is little or nothing written in the scientific literature linking molting frequency and iodine. However, it is noted that toxins are sometimes sequestered in the exoskeleton, which is shed if levels become too high. As iodine is somewhat toxic, this may be the explanation of why crustaceans suddenly molt or molt more often when iodine is being dosed.

Xenia (or other soft coral) needs/ benefits from iodine addition
Again, there is essentially nothing known on this subject. High levels of iodine have been measured in Xenia, but this does not imply any need for it. One of the most common uses of iodine in species that accumulate it is to deter predation. In a predator-free reef tank this is a need that doesn't need to be met.

Changing your water flow, lighting, pH, nutrients, or iodine will increase pulsing in Xenia
The cause of pulsing in Xenia is completely unknown. Changing any one of the factors above may help, but they may not. There is no demonstrated correlation between any of these and pulsatility.

Corals get all of their food from photosynthesis
No coral gets all of its food from photosynthesis. Photosynthesis only produces carbohydrates, which are basically an energy source. Depending on the species and conditions, corals can meet anywhere between 0-115% of their energy needs via photosynthesis. However, photosynthesis does not provide nitrogen or phosphorus needed to build or repair tissues and proteins. This has to either come through ingested food or be absorbed from the water (with the former being more important for most species).

You can feed corals phytoplankton
Very few corals are known to eat phytoplankton. The ones that do tend to be the species that have odd, poorly understood diets and fare poorly, even when given copious amounts of phytoplankton. Dendronepthea is one of the most famous examples. Most corals feed on zooplankton and bacteria. Some like Xenia and some zoanthids are not known to actively feed and fall into the minority that can meet their needs for nitrogen and phosphorus through absorption from the water.

Filter feeders are great to have in the tank because they clean your water
The term filter feeder simply means that an animal gets its food from the water column with the help of the water motion, not that it will improve your water quality as a filter would. Many filter feeders, such as bivalves and featherdusters feed on phytoplankton. Others like corals feed on zooplankton. Neither of these are likely to be polluting most tanks. Things like sponges and tunicates feed mostly on bacteria, something that makes up the bulk of many coral diets as well. The important thing though is that as with all animals, filter feeders produce nitrogenous waste (ammonia). Only a few of them that house zooxanthellae can ever break even, much less reduce nitrogen in the water, and only under certain conditions. The rest increase dissolved nitrogen.

Sponges should be placed in dark areas
No such generalization can be made. A large proportion of reef sponges form symbioses with cyanobacteria and are found in well-lit areas. Placing them in a poorly lit area is a death sentence.

Zooxanthellae give corals bright colors
Zooxanthellae are all golden brown. Aside from the brown color of your corals, they only contribute to the lightness or darkness of the color. The blues, greens, reds, and all other colors are due to pigments produced by the animal itself.

Corals only benefit from a few hours of lighting. After that photosynthesis starts bogging down and productivity drops off, so longer photoperiods are just wasteful
The productivity of photosynthesis is a product of intensity, not duration. Photosynthesis will increase as intensity increases, but at a certain intensity photosynthesis becomes saturated. Beyond that intensity, any increased intensity is wasted or can even cause damage that reduces the photosynthetic output. However, for a given intensity like we have in a reef tank, corals benefit equally for as long as the lighting is on. Studies have even tested 20 hr photoperiods before with no decline in productivity.

The oils from your hands or exposure to air will kill sea stars
There's nothing in their physiology to lend credence to either of these, nor do either get any mention in the scientific literature. They're nothing more than attempts to explain why most sea stars seem to suddenly die in captivity with no apparent cause.

Ich is always present in every tank
Ich is caused by a microscopic ciliated protozoan. It has a defined lifecycle that can be broken by proper treatment of existing infections and of any new fish added to the system. Doing so can ensure that ich is not present in every tank. This myth likely arises from the immune response of fish, which can keep ich at bay for a long time only to have it suddenly appear out of nowhere.

I added (fill in the blank) to my tank and ich disappeared
The only proven methods to treat ich are hyposalinity, copper, or tank transfer. The main ingredients of most "reef safe" ich medications have been tested and found to be ineffective against the parasite. Furthermore, Steven Pro tested the reef safeness of a few of them and found that many were of questionable safety. Garlic, ginger, and pepper laden food have not been subjected to controlled testing.

Hobbyists should be extremely wary of claims that some cure was given and ich was gone the next day and never came back. This is the lifecycle of the parasite. The trophont is embedded in the tissue of the fish for a few days, giving rise to the white spots, after which it falls off and the spots disappear. The parasite has not been eradicated though, it has simply entered the reproductive phase. However, it has been shown that fish develop at least partial immunity to the parasite after the initial exposure. At this point they may only harbor 2-10 parasites. Since most trophonts will either embed in the gills or fail to produce visible nodules, hobbyists are unlikely to see any signs of infection as long as the immune system isn't compromised... whether the fish were treated with anything or not.

A cleaner shrimp or wrasse will help with ich
There is no evidence of this. Ich has never been found among the stomach contents of any cleaners in nature. Nearly all of them feed on mucus, dead skin, and parasitic crustaceans (which are orders of magnitude larger than ich). Also, the ich parasite is embedded within the skin of the host, not on top of it, so the cleaner would literally have to wound the fish being cleaned to access the parasite. The parasite is only accessible to cleaners for the 5-10 minutes that it takes for it to burrow into the tissue. This occurs in the early hours before sunrise, when nearly all cleaners are inactive, so they are unlikely to take advantage of this opportunity. In the only case where cleaners were tested against ich, they were forced to feed during this short period when the parasite was accessible. While some did appear to eat the parasite, none of them ate enough to effect a statistically significant reduction in parasite loads on the fish.

A UV sterilizer will help control, if not eliminate parasites
This is true in single pass applications- that is where 100% of the water passes through the sterilizer to get from point A to point B. Examples of this would be the intake on a pipe drawing in natural seawater or in the plumbing between two tanks of a multi-tank system. In this type of application, UV sterilization can remove as much as 99.999% of pathogens.

However, most hobbyists don't run this type of system. We have recirculating systems. On this type of system UV achieves similar kill rates at the outlet of the sterilizer, however it has only rarely shown any statistically significant reduction of the pathogen load in the main tank. It has been tested against, bacteria, fungi, and protozoans, including freshwater ich (which has a similar lifecycle to, but is unrelated to marine ich) on recirculating systems, of which it was only effective in reducing fungi in the main system. However, even in that case, there was no reduction in the rate of fungal infections. This ineffectiveness on recirculating systems is due to two main factors. First, UV only works against organisms that actually pass through. Any that are surface bound (which included the desirable nitrifying and denitrifying bacteria) will not be effective. Those pathogens such as ich that are only transients in the water column also have low probability of passing through the sterilizer. The other big problem is that the volume of sterilized water is always much less than the unsterilized water which it is being pumped back into. As a result you end up diluting the pathogen concentration, making them even less likely to encounter the sterilizer. As a result, you get diminishing returns and the reduction actually becomes asymptotic even in an idealized theoretical situation.

Salinity of 1.021 or thereabout will reduce stress or the likelyhood of parasitic infection
The idea that stress is reduced by lowered salinity is silly. Most reef fish are fully marine fish that have evolved to live in full strength seawater (above 1.025). At lower than normal salinities their bodies have to work harder to maintain osmotic balance. This can lead to problems with the gills, kidneys, or liver of the fish depending on the species.

As for parasites, lets go back to ich. It is not killed or otherwise deterred by a salinity of 1.021 or even lower. In fact, it is more prevalent among estuarine fish than fully marine fish. Only true hyposalinity, below 1.009 will kill ich, however it will eventually kill fish too.

Besides the effects on fish, hobbyists maintaining low salinities can expect to constantly replace their clean up crews since most invertebrates have very little ability to regulate their osmotic balance and will not last long (which is basically the idea of how it's supposed to kill parasites).

The granddaddy of all reefing myths- Temperature!

The ideal temp for reef tanks is about 77-80
Well this certainly isn't a bad range, but it's not necessarily ideal either. The worldwide average for coral reefs is a wintertime low of 77 to a summertime high of 86. The overall yearly average is 82. The average temp in the coral triangle here reef diversity is highest (and the majority of the livestock in the hobby is collected) is around 82-83 depending on the source. The thermal optimum, which is the temperature where a species grows best, has been tested for a handful of corals and for almost all species falls between 82-84.

Cooler temperatures are better because they give you more margin of error in case of an emergency
The threshholds for thermal damage in corals are set by acclimatization. The rule of thumb is 2-4 deg F above the normal seasonal maximum temperature. Whether your tank normally maxes out at 78 or 86, the corals will still only handle prolonged exposure to about 2-4 degrees above that.

Sometimes oxygenation of the water is cited as contributing to this effect too. While this is somewhat true, the effect is very minor. Increasing the temperature from 78 to 86 only reduces the oxygen saturation point by 7%. That still leaves you at about 300% of the safe level of oxygen. The temperature effect on metabolic demand for oxygen does not follow a clean curve in the sense that you can say higher temperatures demand more oxygen as people often insist. Again, this has to do with acclimatization and can get somewhat complicated.

The temperature on reefs is stable
Not by a long shot. A typical reef varies at least 3-8 degrees per day with some varying as much as 15. Because these were only measured over fairly coarse time periods, it's likely that short-period changes that occurred quickly were missed. These are not slow changes occurring as the sun heats the water either. In fact it has been noted that the minute-to-minute variation is frequently as much as half of the yearly variation. The origin of these fluctuations are shifting currents, tides, and internal waves. As a result, variation actually increases with depth, contrary to what most hobbyists might imagine.

Stable temperatures are essential for healthy corals and fish
This one seems to have originated with studies done in temperate freshwater fish that showed increases in disease when they were exposed to rapidly fluctuating temperatures. The same has not been demonstrated for tropical marine fish, much less for corals. Given the unstable nature of wild reefs, you would not expect this to be true for reef organisms. In fact, it has been noted that larger fluctuations help protect corals from temperature stress.

Great post Greenbean. I'm going to sticky this

Inverts are short lived so expect to replace your clean up crew periodically
Some are, but some aren't. Inverts are far more diverse than vertebrates, so there's no way to generalize. Some animals we keep live a year or two (most slugs), while some snails can live several decades or even a few centuries. Many echinoderms, like sea urchins, tend to be long-lived too, with some of them lasting a few centuries or even being theoretically immortal.

Mariculture is different than aquaculture
Aquaculture is underwater agriculture. Mariculture is marine aquaculture. Neither implies anything about the culture method and since all of our species are marine, they can be considered synonyms as far as the hobby is concerned. When talking about where the culture is taking place, you either talk about ex situ or in situ culture. The former means that it's being done outside of the original location and the latter means it's being done in the natural habitat. To talk about the methods being used you talk about intensity. Intensive culture is what hobbyist do at home where virtually all parameters from chemistry, lighting, and flow are all controlled. Semi-intensive would be like using an open system with natural lighting where only some parameters such as disease, algae, storm damage, and predators are controlled. Extensive would be something like breaking frags and placing them back on the reef to grow, where there is little control over anything.

Moonlights induce spawning
Mass spawning is poorly understood. What is known though is that it takes several signals acting in concert to induce spawning. Having one without any of the others won't work. For many inverts, temperature and tidal changes seem to be important. While corals can sense moonlight and their spawning is synchronized to the phase of the moon, there is no evidence that the actual moonlight itself is terribly important in regulating spawning. This would be a pretty unreliable cue since clouds could obscure the cue the one or two nights a year that everyone is ready to spawn. There also doesn't seem to be any correlation between the presence or absence of artificial moonlighting and captive spawning.

Furthermore, most artificial moonlights on aquaria are orders of magnitude brighter than even the brightest full moon. so probably do a very poor job of simulating moon phases. As a general rule of thumb, if you can read by the light, it's too bright to simulate underwater moonlight.

Crabs with flat tips to their claws are harmless. Crabs with hairy legs, big claws, or red eyes are bad
None of these characters tell you much of anything about a crab. All are common features in crabs of all sorts of temperments. Generally, true crabs shouldn't be trusted in a reef tank. While some might favor herbivory, even those are opportunists, especially as adults. That means that they will eat whatever gives them the most nutrition for the least work.

Probably the only worthwhile rule of thumb with crabs is that those with 8 walking legs are bad and that those with only 6 walking legs might be ok. Six walking legs is indicative of the anomurans, which is the group that includes porcelain crabs, squat lobsters, and hermit crabs, many of which are reasonably well behaved.

Bristleworms are fine until they get big and then they start attacking fish and eating corals/ Fireworms eat corals
The bristleworms are better known by the scientific term, the polychaetes. "Bristleworms" is actually an old common name for the group that predates the hobby. This group includes nearly all of the worms in the hobby except flatworms and peanut worms. Featherdusters, spaghetti worms, eunicids, and fireworms are all included. There's no way to generalize about the habits of such a diverse group.

"Fireworms" is the common name for the family of bristleworms known as the amphinomids. Again, this is an old common name that predates the hobby. These are all of the reddish worms with calcium bristles that most hobbyists recognize as bristleworms. None of these worms have jaws, so they have no way to subdue or eat active prey. The bristles are purely defensive and easily broken (and aren't venomous BTW). With the exception of one species, all of those that make it into the hobby are pure scavengers or detritivores no matter how big they get. The one exception is Hermodice carunculata, the bearded fireworm. This is a large species of fireworm with a prominent red tuft on its head. If you have any doubt as to whether your worm has the tuft or not, it doesn't. It will also be active by day and feeds on sessile invertebrates without fear of predation. This species is extremely rare in the hobby because it's only found in the Caribbean and very little besides fish and inverts is collected there.

Some bristleworms, such as the eunicids may switch to feeding on corals as they grow older, but these are not the common bristleworms (fireworms) and should not be allowed to give the good guys a bad rap.

You shouldn't do a water change during cycling or you should attempt to increase ammonia if you only get a small spike. Doing so will speed up the cycle or allow you to keep a larger bioload later on
Ammonia is toxic and should be kept at a minimum to preserve any life on the rock. Allowing it to climb does not speed things up or increase the future bioload the tank can support.

It helps to think about what's actually happening with the bacteria from a population modeling perspective. When ammonia is measurable, that means that it's in excess for the current bacterial population. Because it's not limiting, increasing the concentration doesn't affect the growth rate. All that's affected is how big the bacterial population can eventually grow.

After the initial die-off is over the rate of ammonia production is much less, so the high bacterial population can't be supported anymore and it dies off. The bacteria don't just hang around waiting in case you decide to add a few fish. Where the population eventually settles at equilibrium is determined by the normal, post-cycle ammonia production, not how much ammonia was produced during the cycle.

Live Sand is agreat biofilter

Although live sand does contain bacteria on arrival, it will only house heterotrophic nitrifying bacteria as the chemoautotrophic strains (Nitrosomonas/Nitrobacter) are long dead. They only survive short periods in transit and only when refrigerated, they cannot form endospores like the heterotrophic strains. Even if the sand does eventually become colonized by true nitrifying bacteria, the water flow between the sand grains is minimal at best and therefore the water passing the bacteria to be "cleaned" is miniscule. The third problem here is that light inhibits bacterial growth and so the the upper layers of the sand, which would have the most water movement are brightly lit and thus impede colonization.

This is great. There are so many jacka.. er.. I mean people that I have been arguing with for years whom I need to show this to. Thank you

The oils from your hands or exposure to air will kill sea stars
There's nothing in their physiology to lend credence to either of these, nor do either get any mention in the scientific literature. They're nothing more than attempts to explain why most sea stars seem to suddenly die in captivity with no apparent cause.

wow....i agree... if exposure to air kills sea stars then every single low tide would have mass die offs in places with rocks and tidepools... in the pacific northwest its not uncommon to see a rock covered with a couple hundred stars durring low tide. same is true of the support pillars on docks.

great thread i hope everyone reads it so we can end the mythconceptions that are floating around!

How Venturis Work

Not sure if anyone cares, but many out there have described the physics behind venturis and downdraft skimmers incorrectly. I usually her that it is the expanding water after a constriction that draws in air. Although this makes logical sense because there should be an increase in pressure before the constriction and a subsequent decrease in pressure after, which draws in air. This is NOT how it works. The force behind it is Bernoulli's principle which states that when the velocity of water increases, due to the conservation of energy, the pressure decreases. So when the diameter of a pipe decreases, the water speeds up and the pressure drops. If the pressure drop is below atmospheric pressure, then air is drawn in, typically at the throat of the venturi where it is narrowest. The faster the flow and/or the narrower the constriction the bigger the drop in pressure.

The reason for the particular angles seen in venturis is to minimize friction loss as the water passes through it.

Nice post spinycheek

Well thank you!

Thank you guys for contributing. I was afraid for a while that this thread died an early death.

This is exactly the kind of stuff to address in this thread. No myth is too trivial. Also, if anyone has questions or comments about any of the myths posted or if you just have questions about whether something is a myth or not, feel free to discuss that here too.

OMG!!!

I have heard some doozies over the years.

Great thread!

That Coral is an SPS and Needs 400W 20,000K MH's

First things first.....there is no such thing as an SPS coral. That is a term that is used only in the hobby. However, that's not the myth I'm talking about. Just because the hobby has labeled all hard corals in one category doesn't mean they all have the same lighting requirements. Quote "SPS" Unquote corals live in different areas.....some are primarily at the crest and some are from much deeper waters. Some do well on patch reefs, some do not. Some do best on the leeward side of a reef, etc. That influences not only the spectrum of light that they receive but also the intensity.

Ahh good point curtswearing.