Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Run the filter on whatever container the fish are in. The fish will need the ammonia removal, and water circulation so it stays well oxygenated. The bacteria will need the oxygen and ammonia to live.
I do not understand what is going on with the main tank.
A fish died. By the next day you could have seen an ammonia spike, but it is a small fish in a comparatively large volume of water. So maybe there was no ammonia spike.
If there was a rise in the ammonia large enough to test, that would have been followed by a rise in the nitrite. This lasts a bit longer, so you might catch that when you test the tank. With that small a mass, though, the bacteria and plants would remove the ammonia and nitrite rather quickly, and the levels should not get toxic anyway.
5ppm NO3 is a low level. I add fertilizer to the tanks when the NO3 is this low.
Fish food gets turned into ammonia (fish digestion and decomposer organisms in the tank). The nitrifying bacteria start with ammonia, turn it into nitrite, then nitrate. For these bacteria, nitrate is the end result.
Now, the tap water issue is something else.
Water companies are out there all the time fixing broken pipes. Often, after a break, they will boost the chlorine or cloramine level for a day of so in the affected area just in case some bacteria got into the water. They do not tell you about it.
The way the water behaved when you turned it on does sound like the water had been off. If you did not do it or know about it, probably the water company did it.
Then, I would suspect the sick or dying fish might be from chlorine or chloramine.