Adding a new fish

[4127john]

So I have a pair of clownfish and a pseudochromis. The lfs said that I can't add any more fish or the fish would be aggressive. Honestly a pseudochromis was the only option with my clowns because my clowns killed many fish. I figured a six line wrasse is pretty aggressive so can I add it with the pseudo

 

[Mebbid]

How large of a tank do you have and what fish have the clowns killed so far?

 

[4127john]

I have a 20 gallon tank. My clowns killed a royal gramma and a bangai cardinal. And also pseudo killed my cleaner shrimp. But now my coral banded shrimp lives in the tank