If you google around the web you will fine that people sell these cheap green plastic chambers from China for $10 to $30 depending on how big a pig the reseller is. they are all pretty much the same, some having one ball, some having 6 balls and twice the length, some having spiral fins, some have a white airstone, some don't, but the key is to get the same plastic top/bottom screw on/off housing with the in/outlet mounted at a tangent, and at least one length of middle housing pipe. The in/outlet tubing will adapt to our 1/2" size which is what most cheap gravel vacuum lift tubes use. If you get one with an airstone, I recommend yanking it out, so you'll have one less item to clean when it inevitably gets clogged over time. with shipping I got my "Up Aquarium Supply" D-402 off ebaY for only $15 just a few Months ago. BTW one "bad" thing about having a high Co2 diffusion rate, is it continuously tries to make your tank water more acidic, so be prepared to keep pH buffering it with baking soda almost daily. Limestone
Calcium Carbonate like Texas Holey Rock my help keep the pH above neutral, but it does not look very natural in a planted tank.
here's the whole cheap Chinese line;
http://www.up-aqua.com/pro-waterplan...nt-diffuse.htm
this Malaysian is selling the D-421
http://cgi.ebay.com/ws/eBayISAPI.dll...m=250129792568