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Help, fish dieing, do salts need to be added to R/O water?

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I have a 55 gallon tank in my apartment and the water looks great but some fish have been dieing.
I'ts my roomates tanks and all he uses is R/O water. Would a lack of salts/minerals cuase them to die? The ammonia content is good, water changes every 7-10 days with a vacuaming of the gravel. has a bubbler and 2 filters. It's a tropical community tank.
thanks for the help before any more fish die.
If the temperature or composition is very different they might be going into shock.
Have you gotten readings on anything else? (Nitrates, pH, etc...)
If all the readings look fine, try observing for some type of disease and/or buy a water conditioner such as prime.

What kind of filters are in use and what type of filtration do they supply? (mechanical, biological, or chemical)
Pure RO water is VERY unstable..
the PH can fluctuate up and down like crazy..

pure RO should never be used as the only water for an aquarium..its VERY dangerous!
at most, it should be mixed 50/50 with tap water..

in reality, he probably doesnt even need to use it at all..
tap water is probably perfectly fine.
yes, fish need the minerals in tap water..

unless your water is SO hard thats its "liquid rock"..which is unusual and unlikely,
your roomate should stop using RO completely and simply use pure-tap..

unless you are breeding discus or angelfish, there is no reason at all to use RO..
and yes, it is probably the pure RO that is killing the fish..
from the fish's perspective, pure RO is pure poison..

Where do you live?
(just city/state is fine)
we can check your local water hardness on-line..

and what kind of fish are in this tank?
I bet he can use pure tap with zero problems..
in fact, pure tap is probably MUCH MUCH better for the fish than RO..

I agree with scottychaos. Pure RO water is very bad for freshwater fish. With the pH fluctuations and lack of minerals that is probably what is killing the fish. Some people will use it for saltwater because the salt that gets added helps with the lack of salt/minerals. For freshwater it is definitely bad news though.
I'd also like to add that I use tap water mixed with Kordon chlorine remover and ammonia remover (two different chemicals), and have had no problems whatsoever for the 3 years the tank has been established.