Well, a canister filter is basically a sealed container, with the exception of the input and output. To also use it as a refugium, you would need to have a completely transparent lid as well as a very simple mechanism to open it up frequently to trim excess growth, which would likely cause an extreme amount of wear on the seal. You'd be better off just having a canister and using a HOB style refugium, or doing a sump and adding a refugium to that. If you want to use the extra stuff, you're going to have to spend more money to do it correctly. The AC 110 has a ton of mods available online to turn it into a refugium. I'm sure something similar can be done with an AC 20, which is in the neighborhood of $20 on Amazon, and can be used either on a sump or on the tank, both HOB style. AC 30 is probably just a few dollars more, but has a noticeable increase in the space available to work with.