Fish worries....?

I have 10 danios, 3 marble mollies and 3 ballon mollies - all of them were doing really well.. until one of the blasted Danios had babies...! We now have 20 babies swimming about too..! It now seems that ALL of the fish are staying at the top of the tank - they are not gasping for air, but they are not going to the bottom of the tank at all. The babies are also at the top of the tank... they all seem OK - all eating and still swimming about... is it because of the babies? I am taking the babies out today - my aquatic shop is taking them. The bottom of the tank is littered with food and faeces - I am getting a suckermouth to deal with this, just don't want to get a new fish in the tank until I know what's wrong..! Any ideas?