I don't have any canisters on smaller tanks. I use an AC20 on a 10g, an AC50 on a 20g long, and an AC70 on a 25g. The only one that I feel is a bit over the top is the AC70 on the 25g, but even then it's not *too* much, just a lot. I have injected CO2 in the 10g and 20g via an Atomizer diffuser, and will be doing the same in the 25g shortly.
To me, a canister is just an excuse to build a reactor for CO2 injection, and smaller canisters often don't have the power to deal with the couple extra feet of head that a reactor necessitates. Though, to be fair, I've only used Rex reactors and never a Cerges or a manufactured model like the Red Sea or any inline diffuser, which add minimal or no extra height for a canister to overcome.
And I agree with VeeSe, surface agitation will help introduce O2 to the water, while the loss of CO2 is really just some extra loss of a dirt cheap gas. Oh, and HOB filters are very quick and easy to clean out.
By the way, the best AC prices I've found have been through Amazon.com
And one final thought - by far, the biggest headache I've found with HOB reactors is increased evaporation ...which wouldn't be an issue if I didn't have 3 HOB filter tanks in my bedroom. I have a deep tupperware bowl next to my bed that I use to top off tanks every couple nights