External parasite issues?

A handful of the approximately 600 smb fingerlings I have in a recirculating tank may be exhibiting symptoms of some from of external parasites. I've got three or four of the largest ones (thought i did a good job of grading them but apparently didn't) flashing against the membrane diffuser plate in the center of the bottom of the tank. No other fish are doing it.

Normally I do a saltwater dip before putting pond fish into the system but forfeited it this time due to the high temps I seined them out of, and what appeared to be weak stressed fish at the time. I was afraid I would kill them in their weakened state. I also accidentally introduced some snails which I have hopefully entirely removed.

My plan is to salt dip all the fish and place most of them all in to a different tank, while the bigger ones will go into a floating cage in one of the ponds with the rest of the fish their size. The larger ones are feeding very aggressively btw in the floating cage 4 X per day. The ones in the tanks are definitely feeding but not as aggressively, however they are not as crowed as the smb in the cage.

Is there any possibility this is not a symptom of parasites? Why does it seem to only affect the larger fish and a very small number of the population?

Is there anything I can treat the entire system with that will not harm the biofilter?

Edited by az9, 06 August 2012 - 10:47 PM.

If there is a parasite to blame and it's flukes then praziquantel will treat them with no effect on a biofilter. You'd have to do a scraping and some microscopy to detemine that. Normally if flashing is observed and there are no visible parasites flukes would be the first suspect.