Fish are sick, don't know why
Things have not been going very well for my tanks lately, I can't figure out why though and was wondering if somebody here might be able to help out. Here is a run-down
Tank: 15 gallon
Ammonia, Nitrite: 0
Light: 15 watt
Filters: Aquaclear 30 and Eheim Aquaball as backup
Fish: 9 danios, zebra snail, MTS
I picked up 6 celestial pearl danios a month ago and only 3 are left. They wouldn't eat at all, got skinny with sort of curved back, then died. There were no real external signs of disease, but they went dark before dying I think.
I put the remaining 3 in a 5.5 gallon tank and one of them still won't eat. The other two look good and are eating better. I used ceramic rings that were in the 15 gallon filter for awhile and added them to the 5.5 filter (Aquaclear 30).
There is now a zebra danio in the 15 gallon which started acting strange a few days ago. He had a sort of lump on his stomaches, as if he had swallowed something. I though he was constipated because he seems prone, but peas didn't do much. He has gradually stopped eating and now the lump seems gone or something. I thought he is a male, but perhaps they were eggs?
I suspect either an internal bacterial infection or some sort of parasite, but can't figure out what. The danio is just not as active (he's supposed to be the dominant male) and doesn't feed. He slowly swims into the current near the top or middle. He's not swimming on his side, or sinking, or any other obvious sign. Please help if you know what it might be. I'm worried its related to the celestial pearl danios dying off, but I don't know for sure.