Hi and welcome.
Your question would likely be seen and answered by more forum members if you reposted this in the tropical fish "new to fishkeeping" forum.
I'm afraid i'm unable to help you, as I don't have the appropriate knowledge.
Though when you say water is perfect, have you tested this with a testing kit? Testing kits are a nescessity to keeping fish, and you'll be asked for test results fairly often on here.