Fish Keep Dying

[Garp]

I have an established 55 gal freshwater tank, lately I can not keep fish alive for more than a day or two. I had two gouramis for a long time, blue one was aggressive. I think he bullies all new fish and stresses them to death. I put in 3 africans yesterday, smaller , to see what would happen, they died. Should I get rid of Gourami, my water is fine.

 

[WILDER]

What is the ph of the store to the tank, how long are you climatising them for, also introducing new fish wihout quarantining them can fetch desease into the tank, any flicking and rubbing on objects, or laboured breathing, anything to go on.