Not sure how this will work for you guys, but I'll tell you what I use in my 10g, and will be using in my 5.5. I have a Fluval 1 internal filter in each of them. It's not very big compared to other filters I've seen. I run my CO2 tubing into the back of the filter, into one of the holes used for attaching the suction cups. I soften the force of the water coming out of the filter by attaching a small T adapter to the outlet and directing one end against the glass. I've thought about making a spray bar attachment too. The filter then shoots out CO2, and although I haven't measured the CO2 levels, it gets my plants pearling. The filter is about $12 online, so I've found it's a nice inexpensive way to get CO2 into these tanks.
30g planted with cories, white clouds, Harlequin Rasboras ,ottos.
10g planted with glowlight tetras and an otto.
Outdoor pond with one common goldfish and comets.
5.5g with endlers.