Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Friends and Eldachleich:
Run some tap water into a glass.
Test pH right out of the tap.
Let the glass sit out 24-48 hours.
Highly likely what is happening:
There are dissolved gasses in water.
Some water does not have as much gas as the air. (Especially well water)
When such water is exposed to the air it gains gases, especially CO2.
Adding CO2 to the water drops the pH.
I do not know if this is killing the fish, but it could be.
Running a bubbler or small pump in the water overnight will help. By circulating the water you are allowing it to adsorb all the gases it can so it will have the same level of gases as the tank water. (This is a good time to add minerals if needed, dechlor, or filter the water through peat moss, or any other treatment it needs, too)
I have sort of the opposite problem. My tap water already has gases in it, and holds more gases in the cold weather, and this causes problems for my fish. I have lost some after doing a direct fill after a water change. I also fill a barrel and circulate the water overnight to make it safe for the fish. But in my case the excess gases are leaving the water.
Other than that, you can test each item in the tank separately, by placing it in a container that just has water (perhaps from another tank that does not kill fish) and a fish. For example, test the rock from the suspicious tank, a handful of substrate, the ceramic mer-person and so on. Test each item separately.
If you find that something from that tank is killing the fish I would suggest thoroughly cleaning the tank, perhaps new substrate, in hopes of removing any lingering traces of whatever is killing the fish.