Fin Rot help?

[Joining the reef]

Ive done a search on here and on google and read a bit but am wondering yalls opinion on my experience so far. First saltwater tank in almost 3 years. Aquarium is 2 and a half months old and cycle finished about a month back. When cycle was completed I added a royal gramma that looked great for right at about a week, then over night fin rot took his entire tail and I lost him that day. I added another royal gramma about a week and half ago, and last night I noticed what looked like fin rot starting on the poor guy too. Question is, does fin rot live in the water? I thought it was within the fish brought on by stress. Tank is the fluval 13 gallon. Salinity is stable with ato at 026. Temp is stable 78-80 throughout the day. Levels are 0,0,5-10 using api (not the best I know) or could this just be bad luck? Or anything else I need to test for? I feed him every other day mix of brine, mysis, pellets. Any thoughts are appreciated. P.S. i do have about 15 frags total that all look great and a few have already shown growth, sps, lps, and zoas.

[bloodhouse]

Did you find out what happened? My orchid Dottyback is losing his tailfin it's looking raggidy and I'm worried.