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Build Your Own Low Cost, High Efficiency Internal Filters

bottom filter

The internal filters described here are simple to construct using readily available, low cost components. They are easily customized to fit any size tank or container, from a plastic shoe box of fry to a planted 20 gallon natural setup.Unlike sponge filters, they do not clog up and stop, or float away when a suction cup stops working. By applying the basic principles of bio-filtration, they can be made to go to work immediately, without waiting weeks for the filter to cycle.

These filters perform both mechanical and biological filtration. Mechanical filtration removes suspended particles by passing water through a porous material that traps the suspended matter. This produces clear water, which may or may not be healthy for fish. In biological filtration, water passes through living colonies of specialized nitrifying bacteria that convert highly toxic ammonia waste to less toxic forms. A filter is considered cycled when enough of these bacteria are present to process the ammonia present in a given tank, keeping it at safe levels for fish and inverts.

Materials needed to construct a filter for a 10 gallon tank:

  • Quart size deli container with lid
  • PVC pipe and tee or elbow to fit
  • Polyester filter floss
  • Scrubby, Easter grass, or other high surface area filter media
  • Lava rock, pea gravel, Aragonite or other heavy, high surface area medium for ballast
  • Drill and razor knife
  • Airline
filter materials
filter materials

Cut the lift pipe and fitting assembly so that the top of the lift protrudes a couple inches out of the lid of the deli container used for the filter body. A tee fitting at the bottom end of the lift is best, but I had an elbow on hand here and that also works. Do not use any PVC glue or cleaner. They are toxic. Just securely push the pieces together and they will hold. Drill a hole for the airline near the top of the lift pipe, and cut a hole for the vertical pipe and some holes for water inlets in the lid of the deli container.

Now add the filter media. Here we use an unraveled scrubbie for surface area and to keep the elbow inlet on the bottom gravel free. Then gravel for weight to keep the filter stationary on the bottom. On top of that is the most important layer: used media from a working filter. This seeds the new filter with live bacteria. Then more floss and pebbles. Plastic Easter grass works well in these as it has a large surface area and can be bought for pennies after Easter and used throughout the year.

filter materials
filter materials

Tank water flows down into the top of the filter, down through the media where nitrifying bacteria reduce the ammonia content, then into the lift tube and up and back out into the tank. The column of air and water rising up in the lift tube create this flow. Nitrifying bacteria are aerobic, meaning the more oxygen they get the better they perform and reproduce. Flow through this filter can be increased by using a longer lift tube, or a lift tube with a greater diameter. Filter capacity is also increased by using a larger container. Cut off two liter soda bottles work very well in 20 gallon tanks.

The air here comes directly out of the open end of the airline. Airstones reduce the bubbling sounds produced by the filter, but also tend to clog up requiring frequent replacement. Even the cut end of an airline can clog, but this is overcome by cutting off the end on a diagonal bias, giving an ovoid as opposed to a circular opening. This diagonal cut also works to keep the airlines in brine shrimp hatcheries from filling with caked salt and stopping up.


[Here summary]This basic filter design is easily adapted to fit most of the small, unorthodox containers so often used to house new killi fry. Here is a mini filter that works in a plastic shoebox:

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Fry Food 911 - Instant Green Water

It is important to have the proper, small live foods on hand prior to spawning your fish. That said, in the real world I regularly get 911 emails like this: "Help! My fish spawned and I have no food for the fry!"

If you're going to have really small fry on hand on a regular basis, you should consider keeping a tank or container of green water going to provide first food as needed. If you're caught by surprise and don't have any green water on hand, there is a way to quickly produce a reasonable facsimile. The pictorial here below shows how.

DIY Fry Filter Design

Note: This design for small filters suitable for fry containers and tanks, and the pictures and text below are compliments of Master Aquarist, Lee Van Hyfte.

Mini tank filter a few twists on an old standby design. Thanks go to Ol Larry Botkin from visit a decade or so ago. Miss ya friend!

  • 1"-1/2" T
  • Used 1/2" PVC cut to 1" long
  • Used 1/2" CPVC cut to 4"
  • The CPVC pipe was slid through the 1/2" PVC to form a collar to adapt to the 1/2" PVC joint.
  • The collar was welded in place.

I utilized the CPVC for the smaller inside diameter than the 1/2" PVC schedule 40. Allowing more lift in the tube.

The T was ground down on bottom and a piece of flat PVC welded in place creating a stable upright platform.

Foam piece will be cut to size and placed in the ends of the T.

Voila mini filters for 1.5gal fry tanks tubs. Now I just have to wait for paint to dry, well PVC cement anyway.

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