When I first made an attempt at this, I took a 20oz MT. Dew bottle, basic clear air line, and an air stone. I drilled a smaller hole than the air line in the cap, and use GE 2 silicone (I wanted the mildew resistance). I placed the air stone under the water fall from my filter, filled the 20oz bottle up a bit over 3/4 full with around a cup of sugar in it, and put in 1/8 tsp of belgium beer brewing yeast. This setup is still running today on my 10 gallon and the only changes I've made are to place a 1 way valve in the line and I switched over to non quick rise bread yeast. I get tons of tiny tiny bubbles from this setup. On my 20 gal I did a pair of 2 liters with the separator, I've had a bunch of trouble with this setup. First my one way valve plugged up, then my separator siphoned back into one of the two liters when it stopped producing randomly (probably had infected yeast). I fixed that, but now my air stone wants to put out one giant bubble every second or so which doesn't diffuse worth a crap. I plan to swap that air stone or make a diffuser soon.