I already know this isn't a good sign, but thought I'd post here anyway since I have zero experience with fish illness. Sorry for the long post, just want to include as much info as I can.
So after a marathon 3 month fishless cycle, I finally introduced new fish last week. First I received 12 Celestial Pearl Danio (CPD) from a breeder. All fish arrived alive and I drip acclimated them to the tank. Tank temp is kept at 75-76 Fahrenheit. Later that day I noticed one fish that looked like it had something white on its side; it looked cottony at first, but then started to look more like mucous. After a couple of days observing him, he started to swim in jerky movements. I removed him from the tank and put him in a 5.5 gal
QT tank with Primafix and aquarium salt. After a few hours he sunk to the bottom and started to roll. He then started swimming again but he was clearly struggling then sank back to the bottom. After three times of this, the third time he sunk down and died.
This was last Sunday. I'm not sure if it's related to what's happening now but thought I'd mention it.
One day after introducing the CPDs, I added 5 Panda Corys and 2 horned nerite snails.
Last night I noticed another CPD who looked fatter than the rest. This fish came this way, as I noticed it when I received them, but figured she was older/larger than the rest or perhaps just fatter. So last night I noticed her pooing a longish thin stringy white thing with a blackish band on the bottom. I've been feeding them various things to see what they like; the breeder sent some food samples with them (sticks, wafers, and some very small pellets; none of the foods are white). I also have NLS Community Micro Pellets and Hikari Algae wafers. I have frozen food that I haven't fed them yet.
I observed the fish last night more and she pooed again, seems to be pooing quite a lot and each time it's a thin, white string. She is also very large, bloated maybe, and looks like she's struggling to swim a bit, as if it's difficult to keep herself from floating down. I also caught her swimming by the outtake of the HOB
filter; at first I thought she was exercising but now I'm not so sure.
Last night I thawed and de-shelled a frozen pea and put it into the tank. All of the fish ate it except for her. I kept it in overnight, I'm not sure whether she ate it at all but when I watched her she did not seem interested in the food at all.
This morning I caught her pooing white again (I also caught a few other CPDs spawning, but that's another topic
). She also looks like there's something sticking out from her underbelly, I can't tell if it's a scale or anything to be concerned about, but it's there.
So what do I do? Should I keep monitoring her? Should I treat the whole tak and if so with what? If it's a parasite or bacterial I'm guessing it's inside the whole tank now and the whole tank would need to be treated. But with the CPDs being so small I'm concerned about overmedicating them. I also need a med that's safe for the Corys and snails. And I'm very concerned about putting anything in the tank that's going to kill all the beneficial bacteria that took me months to grow.
Please help..... I'm a week into fishkeeping and so far it isn't going well.
Specifics on my tank if it helps:
20 gallon tall
Driftwood piece, fake plants
Black Caribsea Sand for substrate
Filters: AquaClear 20 HOB
and internal Fluval U2
Temperature: 75-76 Fahrenheit
I do a 5 gal
water change twice per week