To raise oxygen levels you could use an air pump connected to an air hose with an air stone at the other end placed inside the tank. Or you could place live aquatic plants in the tank.
For the most part, people control oxygen concentrations with aeration and limiting the amount of living things and feed you put into the tank. The more fish you put in, the more oxygen they will require. Also, be sure not to overfeed, since decomposing food will also require oxygen. Beyond that, aeration is the only practical way to control oxygen. You want an air diffuser that produces very small bubbles. The bigger the bubble, the less efficient the aeration (the bubbles contain air that pop at the surface, which means that air is NOT getting into the water!).
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Any kind of water filtration system that pours water back into the tank will get some oxygen into the water. Many aquarists also use an air pump with a tube going to the bottom of the tank that connect to an aerator, like an "
airstone". Some feel that using the aerator all the time is bad, 'cause if the power goes out for an extended time, then the fish will not be accustomed to having less dissolved oxygen in the water. Me? I leave my air pump set to a lower output.
Also, you need to skim off the thin protein layer from the water surface a couple of times a week. I use a strip of newspaper about 8 inches wide, and lay it on the water surface, then pull it off and throw it away. I do this 3 or 4 time. (The protein layer sticks to the paper, and is removed when you pull the paper away.)
Oh, yeah . . . the protein layer is bad because it cuts way down on the oxygen able to get into the water through its surface.
yes, there is enough oxygen in a fish tank. it's in the water, somewhere.
No it's not. An air supply must be used.