HELP! Treating Fin Infection in Puffer
 

Hello all,

I have a 37-gallon FOWLR tank that is about 8 months old. I have one striped fangblenny, black clownfish pair, and a green spotted puffer.

The puffer has some sort of infection on his dorsal and pelvic fins. It looks like it could be fin rot, but also has some brown discoloration at the points where the fins meet his body. His pectoral fins are fine, and he's acting as normal as ever and still eating (hogging) regularly. The other fish are all fine.

My water parameters have never been severely off. I tried to fix the issue with bigger water changes, but no such luck...
Ammonia: 0
PH: 8.1
Nitrites: 0
Nitrates: Currently 20 but has fluctuated b/w 10-30

I did have a green algae bloom on most of my live rock, which I scrubbed off with toothbrush (in tankwater I took out from water change). The problem is mostly gone, though some algae is still on a couple of my rock surfaces.

I normally do 25% water change once every 10ish days...but have been doing bigger ones, closer to 40/50% since his illness.
I tried treating tank with melafix, but that just ****ed all the fish off and made my protein skimmer go crazy.

Now, I have bought API's e.m. Erythromycin, and I think that might do the trick, but I'm really just guessing based off research.
Can I treat my whole tank with this stuff, or will that be really bad?
I currently do not have a quarantine tank set up. I'm not against setting one up if that's the only way, but I'd prefer not to if it's avoidable. Looking for advice on the best approach.
Anyone have any better ideas for solutions?

Here is a link to Youtube with a video of the puffer, can see fins really good here.
https://www.youtube.com/watch?v=hc5qPh6mFBs

Melafix would work if you can put him in a hospital tank (if that's an option?). I had this with my clarkii clown long ago and just treated him alone in a hospital tank. Within two days he was showing signs of getting back. He is still in my tank fully grown now

Don't do anything based on guessing it's a bad thing in this hobby...just my 2 cents. Good luck