Possible tail/fin rot
So I'm stumped. I've got a 55 gallon set up with a wet dry filter and a cannister filter for mechanical filtration. I've got a coral beauty, two firefish, and two ocellaris clowns. About two weeks ago, one the Coral Beauty's side fins decayed almostly completely off. Where the fin met the body, it was white and all there was was a slight stub. I started dosing with Mela-fix the other day, and the fin is now starting grow back. The white coloration still persists however. Now just today, after 2 days of using the mela-fix, a chunk of one of the firefishes tails is missing.
I'm almost positive that the coral beauty's issue was fin decay. Now that it happened to a firefish, I'm concered. My ammonia is 0, my nitrite is 0, and my nitrate is 20. I know fin rot can occur when water quality is low, but there's nothing that indicates my water is off. Salinity is .23 and temperature is in the right range. I'm stumped. Anyone have any ideas on how to treat this mess?
On a side note, I have a new QT setup, and I tried to move the coral beauty into it last week, but she absolutely freaked out. I've got a small underwater filter and a heater in there. Should I put some tank water in there and put the firefish in there to treat? I'm worried I'm going to kill either fish if I move them because of how the coral beauty reacted.
Anythin you guys might reccomend would be so helpful. Thanks