I keep Wondering I have 3 fish in a tank that is 27.5 liters did a water change level but when will the oxygen run out does it get used up quick I don't have an airpump so I'm kind of worried. The fish keep going to the top of the tank like to inaway breath. They are allways swimming near the top as well this has happened since I increased there food supply and did a water change of about half the tanks water what shall I do help please Xx
