Help please, I think its fin rot, but i'm not sure.

[Sheepsnana]

Some people have had issues with melafix leaving an oil on the water surface blocking o2 exchange.

I have three air stones in the tank, so oxygen shouldn't be an issue. They're only on half their capacity and still doing the job.

If I had two fish from the same tank get sick, I'd use a UV filter ...

Is that a no?

[GrahamC]

Since fin rot is mainly a gram-negative infection, and Melafix is not effective against gram negative bacteria, then no.

See http://www.americanaquariumproducts.com/AquariumMedication4.html

If you don't have a UV filter I personally would do daily 25% water changes until the other fish are recovered. ( Like doing daily dressings on an infected wound - you're trying to remove the bacteria if present. ) And watch carefully for any signs of stress.

[alanmin4304]

I personally have never used melafix and never will. You should only treat fish that you have identified a problem on and treat the other occupants if it is a disease which may spread. Finrot is somewhat oportunistic and infects fish that are stressed or have fin damage. I would only treat the others if they are showing signs of it. I would use a bacteriacide like acriflavine or furan 2 which can be obtained without prescription. Wunder tonic is pretty broad spectrum also. Good water quality is your main weapon.

[Sheepsnana]

I'm now pretty sure that he scraped his dorsal fin on the concrete that he loves so much, which would explain why only part of his dorsal fin was affected (until the secondary spot appeared). The other guppy in the quarantine tank has a massive tail for such a small fish, it's healed one split fine, but he's struggling with two.

I won't dose the main tank, I'll just keep an extra eye out for any stressed out fish.