Stats:

• Ammonia-0, Nitrites-0, Nitrates-20
• 55 gallon tropical tank (see sig below for what fish I have) with two 200 marineland penguin power filters (underpowered, but I'm not at full capacity yet fishwise, and my nitrates have been under control). 20-30% weekly water changes. Heater with temp around 78 F.
• symptoms: flashing, erratic darting; no visible physical signs of disease

The fish have been flashing for the past week or so. I first saw it in my albino rainbow shark, and I took him out to a QT tank. I figured it might be flukes or some other parasite, so I did one round of parasite clear (jungle). The next day, I was inspecting the main tank and saw some of the other fish flashing too. Rather than chase one little tiny tetra/danio to place in QT, I decided to just treat the whole tank. I did the first round of parasite clear on the big tank, then later that day (or next day?) with partial water changes before each one. I returned the shark to the tank since I figured no need to separate him if I'm treating everyone.

2 days later, still some flashing going on. I was out of the medicine and went to the LFS to get more, ended up getting fluke tabs instead. Followed the instructions on the box, which said to dissolve 5 tabs (since it's a 55g) in water and add it to the tank. Treat every 24 hours (with a 25 or 30% water change). Anyway, long story short, so far I've done 2 treatments of the fluke tabs (last one was Friday) plus 1 with the parasite clear (so 3 total, technically about 4 for the shark since he got early special treatment). And today, I'm STILL seeing flashing. Is there something else going on? I feel like doing another round, but I don't want to if it's not a parasite problem. I read that chlorine in the water can cause fish to flash, but I always add Prime with each water change (but I added some extra Prime today just in case).

Is there anything else it can be?? Am I using the wrong treatments? I probably should have started out with a salt treatment, and I'm considering that as my next option. I plan to let the meds sit for one more night and do a big water change tomorrow.

Any new additions? I would caution against using a lot of meds with really sensitive fish like rainbow sharks. I'm wondering if the parasites are gone, but the medicine may be irritating him still. Just a theory though. Most scaless fish like that are especially sensitive to most medications, as well as salt.

Other than that, I am really not sure what could be going on. Has anything else strange happened recently? How big is your shark? THey are very skittish fish, and very territorial, so it's possible that maybe he's just irritated about something else.