Fish Still Somehow Off Month After Treating

Akeath · Apr 17, 2026

I feel like I'm missing something here, so I've included pictures. Does anyone see any signs of disease in these fish?

In mid-March, shortly after moving to a new place, I suddenly lost a lot of fish. It was also shortly after adding a couple more Harlequins to my school from a pet store I hadn't shopped at before.

I lost all 7 of my Harlequins, 1 Cherry Barb, 1 Honey Gourami, and 1 Aeneus Cory, a total of 10 fish, all in 3 days. I was very frustrated because only the last of the Harlequin to die showed any symptoms - a case of pop eye. The others were looking totally normal. There was no sign they were even dead except they weren't swimming. No lesions, sores, fungus, or visible parasites to be seen. Just - dead.

I treated with API's Body Cure and API's General Cure to cover the bases. I did mix those after looking up that APIs treatments are made to go together, but once I'd actually done so I had some trouble with the timeline of keeping the medications in because it didn't quite line up. After that I haven't lost anymore fish, but I still feel there's something very wrong with the survivors.

The Cherry Barbs (I have 7 now) are all head standing in place rather their normal schooling and showing off. The Cories are only active when there's food, although they are now under their minimum group number, which could explain some of it. (I've been afraid to add more fish until the ones in there aren't showing any signs of distress.) The Honey Gourami are spending more time at the bottom than usual. It's been a month of this.

It might not seem that weird, but I watch my fish a lot and I know they're not engaging in their normal behavior as much and it's got me very concerned. I'm not sure what to do. I can't tell what's wrong, but I feel like the Cherries wouldn't be head standing if all was well. At the same time I'm watching closely and haven't been able to see a sign of any specific illness for me to go off of with further treatments. It could be just having those two medications was the original problem. I did leave my UV sterilizer on when doing the medications, I'm not sure if that could have messed with anything. I took out the carbon in the filters, of course, and put in fresh after the treatments.

Tank specs: 40 gallon breeder running several years
Filters: C3 and Aquaclear 70, as well as a UV Sterilizer
74 degrees
Stocking: Cherry Barbs, Honey Gourami, Longfin Albino Bristlenose, Aeneus Cories
40% water changes every one to two weeks with Prime as the dechlorinator.
0 ppm Ammonia, 0 ppm Nitrite, 10-20 Nitrates, 7.2 to 7.3 ish pH (I'm right at the cutoff mark of various high range and low range pH tests), 14 gH taken today, 5 KH

GaryE · Apr 17, 2026

The usual thing to say is watch for a bacterial infection, an internal one. It makes sense, but because we don't have labs, it's very hard to prove.

It could just as well have been viral.

I see nothing external, and it doesn't sound like anything that those meds would have touched. Headstanding is sometimes from gas, which could be connected to an internal digestive system infection, but again, all we can do is guess. It could be something that's gone though, but done some damage that will take recovery time. That's the optimistic approach.

TwoTankAmin · Apr 17, 2026

From the Google AI when I input: using a UV when medicating freshwater fish

Why UV Must Be Off During Medication

Chemical Denaturation: UV light can "denature" or break the chemical bonds of active ingredients in medications. This causes the medicine to lose its potency before it can treat the fish.
Toxicity Risks: A major danger occurs with chelated copper treatments. UV light can break the bond of the chelating agent, causing a sudden, lethal spike in ionic copper levels that can kill your fish.
Oxidation: UV increases the water's redox potential, which can accelerate the breakdown of complex organic molecules found in various antibiotics and antiparasitics.

Most sites where I checked on this advised turning off UV when medicating. They say it can have a detrimental effect.

I see no obvious symptoms on your fish. Headstanding is not a good side in species which not do this normally. However, there are multiple potential causes. Ir can be a parasite, a bacteria or constipation. Some fish can accidentally gulp air when feeding which can cause a boyency issue.

Akeath · Apr 18, 2026

I can't believe I didn't think of the UV thing. Luckily I don't use any medicines with copper due to having scaleless fish.

When I looked it up Metranodizole (the active ingredient in my internal/external parasite medication) is light sensitive in general and will degrade rapidly with UV light. The praziquantel (other ingredient) isn't as light sensitive comparatively but the exception is UV-C, the kind in my sterilizer.

The other medicine contained the bacterial medication doxycline (for internal and external bacteria infections), and that will breakdown with UV-C as well, it's entire family of medication types is known for that.

I don't think it's constipation or a swim bladder thing. It's like the Barbs choose to rest at an angle, but they still swim normally at times and especially when they're aware of me (the food giver) approaching the tank they all swim normally until I leave the vicinity.

I'm thinking the next step would be turning off the sterilizer, removing the carbon, and treating the tank again. What medication should I try, given I'm not sure if it's an internal bacterial infection or an internal parasitic infection?