Introducing fish to perculas

[naesco]

Too many snails and no room for conchs. Return most of them to the LFS for a credit. By adding algae to your tank you are simply adding to the bioload.

Sorry to say but the likely cause of the ph spike is the death of one or more of them to starvation. Have you checked your ammonia? Another cause would be insufficient current for oxygen exchange.

You have a nano tank which is what I have at the office. I have two false percs and would not add anything else.

You are on the right track to avoid aggressive fish. Avoid the dottyback for that reason. Avoid the firefish unless you have a covered tank and avoid the 2spot as they have a very poor survival record.
Consider a yellow goby or a passive small fish with a good survival rate.
Thanks for asking for advice before you buy.
Good Luck