Excessive Flashing, No Idea Why

So I've got a 75 gallon native tank with fathead minnows, longnose dace, crayfish, and some ramshorns. It's been running for a few months (the dace were separated into two 10 gallons before this, which themselves were also running for a few months), and I've experienced 0 deaths. The tank is generally around ~15°C, and while my ammonia reading is not ideal (~0.4 ppm), it's not terrible either. I do a 10% water change every week and vacuum the substrate (sand), the tank has appropriate-sized filters (more than one), and there are a few plants as well.

Ever since I got the dace, they have been flashing excessively. A few weeks after obtaining them, they developed ich, which I chalked up to the cause. I treated them using the salt method, and haven't seen any ich since (and that was months ago).

However, they've continued to flash, and it seems to just be getting worse. It's also spread to the other fish (fatheads). I thought maybe it was gill flukes, so I treated with praziquantel for 3 weeks (PraziPro, which was hard to get in Canada for some reason), one dose a week, as I am aware of the life cycle of flukes and that one treatment is rarely enough. It did nothing, and the flashing has continued.

My question is, where do I go from here?

I'm thinking maybe of switching to a different medication, or trying PraziPro again, but for longer this time. I'm not really sure what else to try, though. What are some other common reasons for fish to flash?

I've noticed that some of them have rubbed off the scales around the base of their fins from flashing so much. This picture hardly shows the extent of it, though it does indicate the general area:




My thoughts are that either their gills are itching and this area gets scraped in the process, or the base of their fins are itching. Perhaps there's a parasite that affects this area, I'm unsure. I'm hoping someone might have some ideas.

 

Or maybe since they are wild (right) or maybe even not, theyre flashing from the ammonia in the tank. its not a huge amount of ammonia, no, but theyre prob sensitive fish (I'm not familiar with them) id stop treating and just do water changes more then u normally do.

 

Probably the ammonia. If the water’s alkaline it will be very toxic to them.

 

Or maybe since they are wild (right) or maybe even not, theyre flashing from the ammonia in the tank. its not a huge amount of ammonia, no, but theyre prob sensitive fish (I'm not familiar with them) id stop treating and just do water changes more then u normally do.

Right, they are indeed wild-caught. This is a good theory, though I can't say I'm feeling confident about it. The water I got them from is really gross to begin with, and these particular fish are supposed to be especially hardy.

I've noticed that they also twitch/flick their pectoral fins, suggesting their fins are itching. Is this a normal symptom of ammonia-related irritation? Or perhaps there's a parasite similar to gill flukes that causes their fins to itch? I'm thinking some Gyrodactylus species, but if it was just flukes, the praziquantel should have worked... assuming I don't have some weird resistant type.

Edit: I was just reading about a specific Gyrodactylus species on Wikipedia, where it is stated:

Monogeneans also have the ability to survive praziquantel treatment because they can be protected by mucus that the fish produces.

So maybe that's why it didn't work. If someone with fluke experience wants to recommend a better medication for this, I'd love to hear it.

Probably the ammonia. If the water’s alkaline it will be very toxic to them.

That's certainly something I need to check... I need to get a better test kit. Even so, I'm still skeptical of this being the cause. When they were in the 10 gallon (I had about 5 of them in there), the ammonia was a clean 0, and yet they still flashed quite a bit.

 

Try worming them (especially as they’re wild caught). Flashing is also a symptom of intestinal worms.

 

I'm a big fan of Prazipro but I get the impression people prefer something like API General Cure (prazI + metro)
when internal parasites are suspected. Or they start with prazI because it's gentler but switch to the combo antiparasitic if needed. Just something to consider if you want to try a different med.

 

My lfs always recommends to use medicated food as a treatment. He has a fenbendazole /levamisole /praziquantel flake food program that works pretty well.

The ammonia level should be at zero. I've noticed that when I had minnows, they would flash if I didn't do enough water changes . The ammonia may be irritating your fish enough that they're trying to find ways to get away from it.

I'd try doing a 30% and see how that goes.