An aquarium sump filter, or simply sump, is basically a complex filtration system used to maintain the quality of the water within an aquarium. Unlike standard aquarium filters, sump filters do more than simply filter the water. Sump filters make it easier to maintain the chemical balance of the aquarium water, and they help bring down the amount of maintenance an aquarium requires by cutting down the need for constant water changes. Aquarium sump filters are good do-it-yourself projects because they are not difficult to set up and don't require too many materials.
Divide a 10-gallon aquarium (or larger) into three compartments using two plastic aquarium dividers.
Fill one of the end partitions of the aquarium with aquarium rocks. Use porous rocks instead of aquarium rocks, if you can purchase them. Porous rocks can be purchased at pet shops or online.
Fill the partition in the middle of the aquarium with charcoal rocks or the charcoal packets used for standard aquarium filters. The packets are easier to find, but may be more expensive to purchase enough of them to use in a sump system.
Insert an underwater aquarium heater and a water pump into the final partition of the aquarium. The aquarium pump should be large enough to connect to a PVC pipe rather than a plastic tube.
Attach a PVC pipe from the end of the water pump that will be pumping out the water to the aquarium that will be using the sump filter you are making. Use a PVC plumbing kit if you need to add joints or extensions to the piping.
Insert a water pump into the aquarium that will be cleaned using the sump system you just made and attach a PVC pipe from that water pump to the first partition of the sump filter in the same fashion as the PVC pipe was connected in step five. Water will come to the sump filter via this pipe, get filtered mechanically by the rocks, get filtered chemically by the charcoal, get filtered biologically by the ammonia eating bacteria that will grow on the rocks and charcoal within the next 24 to 48 hours and finally get heated and sent back to the original aquarium.
Note that sumps help cut down the number of water changes you need to do, they don't eliminate the need for water changes.