What's your nitrate level? Might need to do up to 50% PWC
if they're really high. Then retest nitrates and make sure nitrates are below 20. Then after you do that, pick a fish, acclimate it and add it!!
thats the really fun part!! only add a fish every two weeks or so, otherwise be adding too much ammonia for your new bio filter to handle.
Your goal for nitrates is of course 0, but everything I've ever read says below 40 won't harm fish. mine are usually around 10 and I've not had any problems with algae or cyano
and nothing has died.