I have kept fish for many years and recently transferred to larger tank keeping old water in and treated new water, there were nothing but platties in. Added six tetras about three weeks ago (1 died) and 5 mollies two weeks ago(3 died suddenly). Now the platties are sat at the bottom or hiding under or in the ornaments. Sometimes they are trying to swim like mad but getting nowhere, tried turning off the filter in case it was too fast but it makes no difference. Noticed a bit of white spot tonight which I will treat tomorrow. Anybody explain the odd behaviour please?