I'm not sure if it's fin rot or something else. New to treating disease.
tall tank, cycled, ammonia 0, nitrite 0, nitrate 0, temp 73deg, pH 6.6. HOB
filter (with active bacterial mat), plus sponge filter on airstone (no visible bacterial mat).
There are 12 serpae tetras (ie red minor tetras) in the tank, various sizes and ages.
One of the small ones has a blackish bulge at the base of its tail (both sides of tail) with white growth-like spots on it. The tail is almost gone (nipped or rotted). There are also extensive white growths toward the front of the dorsal fin. It is eating, but not much. Swimming pretty normally, sometimes with school, sometimes alone at bottom. I noticed these symptoms Friday (2 days ago). I noticed some fin clamping a couple days earlier. I haven't seen any nipping.
Another medium size fish seems to have the beginnings of the symptoms at the base of its tail - a grayish-blackish bulge at the base of its tail.
Tank history: has been cycled for several years. For a long time (about a year) we only had two serpae tetras in it. The plants started to wither during this period - lack of fertilizer maybe.
2 months ago we added 10 more serpae tetras. (From a reputable shop and we don't have a good hospital tank so we just acclimatized them, no quarantine) We also added a mystery snail and a new plant, and 2 mystery snails in our 2.5gal tank (which we eventually want to cycle and turn into a hospital tank).
All have been healthy. Some initial fin damage which healed up, and they started schooling nicely. New plant has been dying though.
1 month ago, we put all of the mystery snails together in the 15gal tank, along with a boiled cuttlebone.
We've done a couple 15% water changes and some vacuuming since then but we haven't tested chemistry for a month until today.
We feed fish TetraMin flakes and PlecoWafers for snails. They've been getting that food for six weeks.
When we noticed the disease, we moved all three snails into the 2.5 gal
tank, did PWCs (about 25%) on the 15gal tank on Friday and today, and started a regimen of Melafix on the big tank. Also added aquarium salt to big tank.
Over 2 days, no improvement, but no worsening either. I tested chemistry today and the pH was acid (6.6) for the first time since we cycled. It's usually 7.2 - 7.6, and it was 7.4 a month ago when we last tested. The tap water is 6.8 today (it was 7.1 a couple years ago when we first moved in).
What would make the tank turn acid? The cuttlebone? The Melafix? Or maybe it's just that the tap water is more acid and I usually don't test pH right after a PWC
. The small tank is still 7.6.
If it gets worse, should we use an antibiotic? Should we isolate sick fish - and in a net cage or separate tank?
Thanks for your help!