Advice for tail / fin rot?

[someginger]

I'm not sure if it's fin rot or something else. New to treating disease.

15 gal 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!

 

[someginger]

PS - the sick fish seems a little bloated too

 

[Delapool]

That looks pretty much what I had with some fish that had columnaris. I’d shift both fish to a hospital tank ASAP and treat with antibiotics. Especially as a second fish is developing same problem. Was the tail lost and all this developed in two days?

Melafix and salt (unless perhaps a salt dip) won’t generally cure it. These can be useful to clean-up/reduce infections but I’ve never cured the underlying infection using them. Some people have, never for me.

I’d suspect the ph drop is from adding extra fish. The extra bacterial activity to deal with waste output would also consume kh so ph tends to lower (pretty natural and unless very soft water, extra water changes will help).

Just as a side thought - extra water changes reduce organic load so definitely clean water helps. Keep temperature about the same. Higher temperatures can make bacterial infections worse. In theory, more acidic water will reduce bacterial activity but water changes to reduce the organic load is more important imo.

 

[someginger]

Thank you Delapool! Yes I only noticed it in two days, but I might have missed the early warning signs. Since then, no improvement, and maybe a slight worsening. We're getting KanaPlex and Furan (fungal and antibiotic treatment) tomorrow. We were thinking of treating the main tank since it seems like the bacteria would be in the tank and the others might be infected (no symptoms yet). Do you think that's a good idea or should we move them into the hospital tank to treat them?

 

[Delapool]

Does sound bacterial infection then unfortunately. Furan + kana - may need a second treatment course.

The furan may kill off any plants. Not sure on the kana.

I’d treat main tank but move those two and any others that look unwell to a hospital tank and treat there. Bad ones I’d put down.

 

[Delapool]

Unfortunately with this I’ve had it work it’s way through entire schools and kill off over a month or I just gave up on treatment.

After treating, just in case - don’t buy any fish for a month to let tank settle.

Older tanks I find have nitrifying bacteria more resistant to meds but keep an eye out for ammonia spikes. Good luck with them.