I have a 20 gallon tank. I don't have any air pumps. Recently, it's been hit where I live, so my tank has been staying around 80 degrees. I know that the warmer the water, the less oxygen it can hold. My filter is a Grreat value hob filter, with a 90gph rating. It creates a little waterfall. I have 5 guppies. Do they get enough oxygen? They don't go to the surface and gulp air, but I still don't want to not give them enough air.