I am new to aquaria and have two tanks: a 10-gallon and a 55-gallon.
Specs on 10-gallon: 3 months old, live plants, UV lights, no sunlight whatsoever, Tetra filter 5-15, temperature is 77, three neon tetras, 1 cory cat, 1 male swordtail.
Specs on 55-gallon: 5 weeks old (but bought used so it cycled almost instantaneously), plastic plants, incandescent light, no sunlight, Penguin 350/double bio-wheel filter plus additional air stone for more aeration, temperature is 78, two baby angels, 1 female betta, 5 zebra danios.
I religiously change 25% of water every week on both tanks. I have never had an ammonia, nitrite, or nitrate spike on either tank. Ph is about 7.8-8.0 on both.
I bought a female red velvet swordtail for the 10-gallon tank. Day 1: healthy, though small. Day 2: stopped eating, clamped fins, thin white feces. Day 3: developed tail rot; moved to hospital tank and treated with Mardel fungicide. Day 4: slightly swollen left side; sinking to bottom of tank, listing to side, extreme lethargy. Day 5: dead.
I then bought four more red velvet females for my 55-gallon tank - all were large, beautiful, healthy, and active. The EXACT same symptoms occurred to one, and then another and then a third of these beautiful specimens!! I moved all fish from the 10-gallon into the 55-gallon and then put all four sick red velvets into the 10-gallon and made it a hospital tank. I've used Maracyn 1 & 2 and Mardel fungicide, again without results.
Three of the four are sunk on the bottom of the tank, unable to move. I need to freeze them, I think. All other fish are symptom free, including male swordtail. I don't get it.
I am assuming that the root cause is bacterial infection since they do not have ich, dropsy, visible parasites, etc. Any help, suggestions, info is greatly appreciated! I don't want to buy any more swords until the problem is remedied.