I’ve purchased a corner filter that can run 2500 cubic centimeter per minute powered by an air pump. I’ve learned that you need an air pump that can power a tank at least 2 times its capacity (e.g. air pump that can filter 50 gallons for a 15 gallon tank). There are two settings for the air pump: hi and low. The filter medium is coarse white foam designed to trap the particles, food and wasted suctioned by the air pump. The said filter medium is to be replaced every week, but I’ve replaced mine after two days because the filter turned orange quickly. We have plans to upgrade the filter in line with the upgrade of the tank; a better option is the motor powered-pump or overhead filter.