Create an Undergravel Filter for Your Fishtank

Undergravel filters, some love them, some hate them. But for less that $30 you can build a biological/mechanical filter for your 5-1/2, 10, 20, or even {{safesubst:#invoke:convert|convert}} tank! If you've got more money, and more time, you can build an UGF for any size tank.


  1. Know the size of your tank. Measure its length width and height.
  2. Plan. Try to think of a design that would allow the PVC to make a closed circuit, and cover a good amount of area. Also leave about a half inch from the sides of the tank and the PVC tubing, as this will allow your substrate to fill in the gap hiding the UGF. Making a closed circuit simply cuts a few dollars off the price of the build by not having to buy caps. Double check to be sure.
  3. Go out to your local hardware store and pick up lengths of 1/2" or 3/4" PVC. My local store sold it in 10' lengths, and it cost > $2.00. 1/2" would be best for 5 1/2 - 20 gallon tanks. 3/4" would be better for 30 or more gallon tanks. Make sure you have all of the elbows and T connectors you'll need. Take into account the lift tubes, which will need 1 T connector for each lift tube
  4. While at the hardware store, ask the clerk where you can get PVC cutters, I got a decent pair for $6.87. Try to get the kind that ratchet, it will make cutting easier.
  5. Once you've got the filter pattern planned out, begin cutting and assembling the pieces outside of the empty tank.
  6. After all pieces are assembled, use a hammer and nail, or a power drill to punch holes into the bottom of the PVC pipes. This will allow water to be drawn into the UGF as it operates. Be careful not to hurt yourself, or go through the other side of the PVC pipe.
  7. Now take the completed UGF and put it into the fish tank. Make sure the uplift tubes are not sticking out of the tank.
  8. Run the air tube(s) down to the bottom of the uplift tube(s). For PVC pipe 1" or greater in diameter attach an air stone at the end of the air tube and drop that into the uplift tube.
  9. Using proper techniques, finish setting up the aquarium (washed gravel, de-chlorinated water, additional power filter (if desired).


  • Have fun and enjoy your aquarium.
  • Keep in mind that the bacterial colonies that develop will only remove soluble wastes from the water itself, and not solid fish waste from the gravel. This will result in cleaner, clearer water however regular substrate maintenance is still recommended (10% water change weekly or bi-weekly, gravel siphoning to remove solid waste).
  • UGFs work on the principle that drawing water through the substrate (gravel) will deter stagnant conditions in the gravel bed and allow beneficial bacteria colonies to develop, furthering the biological filtering capacity of the tank.
  • Avoid overstocking the aquarium. Most freshwater fish need approximately {{safesubst:#invoke:convert|convert}} per inch per fish. So a 3" long pleco would need {{safesubst:#invoke:convert|convert}} of water all to itself.


  • There are many different points of view on the usefulness of an UGF. Take time to consider both the pros and cons. If you have the opportunity, it would be far better to try an UGF on a smaller tank first if you're not sure.
  • UGFs will absolutely not work with sand substrates.
  • If you are not yet comfortable with the idea of an undergravel filter, or still new to the fish keeping hobby please be absolutely sure that you can complete a DIY aquarium project. If you do not know what you are doing, use the wrong materials, or do something wrong you may end up polluting or poisoning your aquarium, or further complicating matters.

Things You'll Need

  • You will need -
  • PVC pipe of the appropriate size
  • PVC pipe cutters
  • PVC pipe fittings (elbows, T sections/connectors)
  • Hammer and nail or an electric drill.
  • Aquarium air pump and air tube (if using 1" or greater diameter PVC pipe airstones may be used in the uplift tubes)
  • Caution with power tools. It is always recommended to have supervision and use appropriate safety gear.

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