Why are my fish always the the top of the tank? ?

I have a Betta, 3 guppies and 5 white mountain minnows. The betta is as happy as can be swimming around everywhere but the hippos are always at the top and only 2 swim down for a bit then go stay at the top... The minnows don't worry me as much since I have read that they like to stay at the top. Now I know it might be lack of oxygen but I just lowered the water level so the filter is causing a disturbance on the surface. They've only been in their for 24 hours.