Blue dwarf gauromi

Question
Hi Erika,
I just have a few questions for you. I have a blue dwarf gauromi who has developed a red spot just before is tail fin on his body. It looks almost like an ulcer it is red in color. I went to my local pet store and showed them a picture of him and they said it was normal. I was not to sure about this, and seeing as I have 15 fish in my tank I want to be sure that this is normal. My last question for you is also tonight I bought 1 other dwarf gauromi that is powder blue but has orange stripes on him and also two rainbow fish. My blue dwarf is being extremely aggressive to them and I am afraid he will kill them. I put him into a nesting box in the tank to seperate him. Will this work and if so how long will it take for him to chill out and not harass the new fish? P.S. I already tried the rearrange the decor trick and it did not work.
Thank you so much for any help you can give me.
Holly

Answer
Hello Holly.
  I think your fish has a bacterial infection caused by aeromonas or pseudomonas. All you have to do is keep him in the nesting box and give him medication containing antibiotics like Melafix, Anti-Bacteria, Fungus Clear, E.M. Tablets, Triple Sulfa, Maracyn, Maracyn-Two, or Anti-Bacteria Medicated fish food. As for you aggressive blue gourami (aka 3 spot), I've heard you shouldnt mix the big gouramis (Trichogaster scientific name) with dwarf gouramis (colisa) because the gouramis from the species Trichogaster are too aggressive so I'd suggest you move your dwarf gourami and rainbows to another tank so they dont get injured. Hope this helps! :)
          Erika