Fin rot help :(

[Joiningsession]

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

 

[Humblefish]

Fin rot can be caused by harmful bacteria (which does go waterborne) eating away at the fins/tail: https://www.reef2reef.com/threads/bacterial-infections.191511/

Or it can be caused by skin flukes: https://www.reef2reef.com/threads/flukes-–-general-guidelines.224423/