Introducing fish to perculas

[Pier Pressure]

You should maybe get rid of the conches as your tank is too small to give them enough to eat for very long. They will starve to death eventually.

And you are looking at putting five fish into a twenty gallon? That seems pretty high to me. I have five fish in a 28 gallon and would only go with three or four if I was to do it again. One is a yellow clown goby so I do not really count him as he is so small. Bioload is extremely high regardless and we do not dare miss a weekly water change of about 20%.

I have had no trouble with my clowns (one real and one false perc) being agressive to any other fish.

No idea about your pH so hopefully one of the more experienced members will chime in.