Basically the same idea as an external DIY canister filter, except you don't have to worry about a tight seal on the intake/outflow points.
It's based on a combination of an external CO2 reactor and Tom Barr's internal venturi reactor.
- Clear, plastic peanut butter jar (any clear plastic container will do)
- Airline tubing
- Drill + drill bit
- Filter media
1. Heat the side of the jar near the top until it softens. Be careful not to burn a large hole through the plastic because you want a tight fit into the powerhead outflow.
2. Fit the powerhead's outflow through the part where the jar was heated up.
3. Drill two holes slightly narrower than the airline tubing's diameter through the lid. One hole is for CO2 line, the other for the air/burp loop
4. Drill a few holes through the jar (these holes are for the outflow of water, so drill them where you want water to flow)
4. Fit a sponge over the intake if you want.
5. Cut airline tubing so it is long enough to go from the airline hole to the hole on the lid. Cut one end of the tubing at an angle so it can be pulled through into the jar.
6. The other airline tubing is for the CO2.
That's about it.
Missing is the actual venturi part, but that can be easily done.
You can also put filter media inside the jar if you want.