Hi All

I'm transitioning a 210G tank to a reef tank in my office. This marks a return to salt water tanks after a 25 year absence. Things have progressed a wee bit.

In effort to accelerate my learning curve i opted to use a maintenance firm from a solid salt water aquarium shop to help me transistion the tank. Simultaneously i have been reading and researching the marine aquarium forums, and learning a bunch. I am a scientist by training and have a solid understanding of water chemistry, and aquatic ecology.

The guys assisting me on my tank are young and obviously experienced. They also are also not very technical, which makes me a bit sceptical. Im not replacing them yet, but i am not totally comfortable with their decisions either.

In doing my research one idea i read was the concept of keeping a portion of salt water equal to your tank volume, mixed and ready to use in water changes and in the event of an emergency. I in turn decided to keep around 200 gallons of salt water (at temperature, oxygenated, and mixed via a powerhead) as well as 50 gallons of RO water (for topping off).

My maintenance guys are 'against' me storing mixed water, saying that salt water 'goes bad' after a couple weeks. Instead they want to mix 1 weeks worth of water change water at a time. Meaning i have 20 gallons or so on hand in case of an emergency.

I'm looking for educated opinions. How and why would mixed salt water go bad? Is the idea of keeping 200 gallons of water mixed overkill?

thats a pretty good question right there. the only thing i can come up with that might back up what they said is that as it sits, with salt creep and evaporation, maybe the trace elements go with it, making the mix not as up to par as fresher mix. i have a 40g tank and i have 5 gallons mixing for a week before water changes and one 5 gallon of plain RO/DI water for top off from evap.
but 200 gallons? not sure if its over kill... really good to have on hand for less frequent mixing before each change. but if my idea is anything to do with their opinion, its not like you can't supplement the water with trace elements to keep it up to par.
but can't say i've heard of it "going bad". if you let it sit for years, sure, under the same idea of sitting water in any thing. but if you use it over a few weeks, i dont see any harm of it going bad.

No, it doesn't go bad. I've heard this before, and it appears to be based on nothing. If you let insects die in it and let it go stagnate, than yeah, that would make it go bad. But if you keep it sealed from dust and bugs and keep a pump in there to keep it from stagnating, then it will last indefinitely. Now, something to consider, if it's in a plastic tub, theoretically over time the water could accumulate plasticizers leaching from the plastic, but I've never seen solid proof that this causes problems, nor have I seen a quantifiable amount that is leached. Typically it leaches more in acidic water, but seawater is quite alkaline. To avoid this potential problem, use a HDPE (High Density Polyethylene) storage container, it's definitely non-toxic as it's approved for food storage and is used frequently in aquaculture. Also consider that an aerator will introduce dust into the water, so opt for a submersible or inline pump to keep it flowing, and as long as the lid isn't air tight, it shouldn't go anaerobic. Some people worry about bacterial slime that tends to build up on the sides of storage containers, but it is not harmful and the aquarium will undoubtedly host much, much more bacteria on its surfaces.

Of course, it wouldn't hurt to test the stored water periodically, just to make sure nothing gets out of whack.