I am just going to go through all the information that the sticky at the top of this section said to provide, so feel free to skip what you think is irrelevant.
I have a 29 gallon tall freshwater tank. Ammonia is .25 ppm, nitrate 0 ppm, nitrate 5 ppm, ph at 7.0, all as measured with a liquid test kit. The temperature is 76 degrees.
I did a fishy cycle, eventually with the help of Tetra Safestart, with 6 cories, one about 1/2 inch, the rest 1 - 1.5 inches. The tank took about a month to cycle, has been cycled for two weeks, and had 3 black-skirt tetras (a little less than an inch long each) in it for one. The ammonia went up again from 0 after adding the tetras. I have plans to add four more black skirt tetras when I get this disease thing sorted out.
I did not quarantine the tetras when I got them, but did put the sick one into quarantine when I noticed symptoms.
The tank has a gravel bottom, several plastic plants, four large ornaments and a smaller one, and 8 java ferns. It is filtered with an aquatech filter for 30-60 gallon tanks, but I'm not sure what sort of heater I have; I bought the tank at a garage sale and it came with it (I cleaned everything with rubbing alcohol before using it).
Similarly, I don't know what lighting I have, but I keep the light on from about 9 am to 9 or 10 pm.
I have been trying to do 30% water changes once per week, but with the ammonia going up I have done two in the past week. I have been using Seachem Prime, and I admit that might be part of the issue, because I have a tough time eyeballing the amount for 10 gallon water changes. I have not been vacuuming the gravel to avoid disturbing the plants.
I was feeding the fish a generic brand of flakes, which I discovered two days ago had some lumps of flakes in it (I had had it sitting around for a few months). Since then I bought Tetramin flakes and have been feeding those.
I noticed symptoms in the tetra that died the night before last when I got home at 8 pm, and it has been dead for a couple of hours now. It was swimming near the filter, apparently having trouble staying in one place and tilting over to one side sometimes, its head pointing up a bit. I waited a day to see if it was constipation issues (and I could not get it to eat any lettuce), then put it in a 1 gallon, bare bottom quarantine tank filled with water from the same 29 gallon, filtered but not heated. It would not eat, and did not move much, but seemed more able to stay upright with the smaller filter. This morning it was face down vertically against the bottom, and over time ended up almost completely on its side on the bottom, then tilted upward on the top, which is how it died. When I noticed it was dead it looked like there was grey goo/fluff sticking out from its sides, but I think that may just have been it rotting or something (although I don't know how quickly that would happen)
The fish seemed a little bloated in the stomach area, but not pineconing at all. Aside from the swimming problems, the only symptom I could find was a bloody spot right in front of its pelvic fins. Another fish is possibly now showing symptoms- it occasionally darts up a bit when swimming, then down, and seems to have a tendency to turn around a lot. It is getting picked on by the other tetra, and seems to be able to move away just fine when it gets nipped.
Finally, and I'm not sure if it's relevant, the dead tetra had a large, speckly black-grey spot (half a centimeter in diameter on a 1 inch diameter fish) on one side when I bought it, which did not fade when the rest of the fish's coloration faded from stress while it was first placed in the tank. I am thinking this is an ammonia burn which it got at some point in the shipping process, but I'm not sure. The other two tetras have some black spots that didn't fade with their stripes, but they are more like small black specks.
What is going on? Is there a way to treat this, and if so how? And also, is there anything I can do about the increase in ammonia besides a lot of water changes?
Thanks for reading this far, and for your help!