I need some advice from you experts.* I have a 20 gallon tank that I have started.* Tank has been active for 5 weeks and added my first fish last week.* Everything was fine the first day then it went down hill.* I have 6 fish 3 mollies and 3 platys along with one cory catfish.* 3 of the fish are staying on the top and 3 are on the bottom, mixed between the two types.Temp 76-78 GH 30 KH-0 PH-6.0 NO2-0 NO3-0 Ammonia .25 I use two filters* Marineland and Topfin 20 and a 4" air stone.What am I doing wrong.**