hmmm this is always a problem when i have very young juvie pbasses...whenever i do big water changes once on my pbasses tank one will die...thats how I lost my Cichla Pinima =( Also your Ph seems abit too high...you should try adding a driftwood into the tank to drop the Ph under 7.0...maybe its from Ph shock that killed it one by one? Its a possiblity as that was what I found that killed my youngins back then...
my driftwood and water conditioner called Prime didn't work fast enough I guess
I only did a 30% though and yet its gone...Damn lil pbasses...why do they have to be so fragile at that small stage? I have had baby cichlid fry's go through a 90% water change back then and they survived with no water conditioner...