Need some help. I have a 29 gallon planted tank with a cheap 265 gph
cannister filter, T5 lighting, no CO2
(at the moment). The tank has been going steady for about 5 years with monthly water changes and cleaning filter media 4 times per year. It has mainly native fish; a small blue spotted sunfish, 3 florida flag fish, 3 blue fin killi's, a couple of dwarf Cajun crayfish and an otto. Last week I received a shipment of 8 additional Florida native fish. All very small, including two juvenile (< 1") long ear sunfish. Everybody was well and eating good (frozen mysis or bloodworm once per day, some flakes for the top feeders) for the first week. Last night I noticed that a small section of the dorsal fin of one of the long ears had turned white. Not fuzzy and not frayed and the fish seemed active and fine. Coming home from work today the fish was floating sideways near the bottom, more dead than alive. It had lost most color on the back half of its body, caudal fin appeared to be almost gone. It barely moved when I scooped it out, and I euthanized it by putting it in the freezer. I already dosed the tank with Melafix, which is the only med I have at hand.
Attached are pictures of the healthy fish, which I got at the same time, and the dead one.
What could it be, and what should I do?