I would hold off buying any new fish and get a test kit if possible. You will want a kit that tests ammonia, nitrite, and nitrate. Liquid based test kit over the test strips if possible.
If you cannot get a test kit or get your water tested properly, then at minimum perform water changes each week. That will keep the ammonia and nitrite down as it is cycling. Without testing, however, you will not know when cycling has been completed.
The filter you have is basically a powerhead (the pump) connected to a square sponge. The sponge will serve as both the mechanical and biological filter. Do not clean it too often. Probably when there is decreased output from the pump. And when you do clean it, DO NOT use tap water; use water that has been removed from the tank after a water change. The chemicals in the tap water will kill the beneficial bacteria (BB) in the sponge. This will undue the work you have done for cycling the tank. It is the BB that consume the ammonia and nitrite from the fishes waste. It would not hurt to get a second filter in a tank that size. A hang on back (HOB
) style filter should do fine. This will help ensure that there is plenty of space for the BB to grow.
Connected to the end of the powerhead/pump is what is called a Venturi. This has a tube attached that has an end above the surface. Water rushing through the outlet of the pump draws in air from the tube and mixes it with the water being pumped out thus producing the bubbles. So technically you are just introducing air, not just oxygen.
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