No harm done, L_L_F (hope you don't mind the abbreviation
). Enthusiasm is good, but so is keeping reponses limited to the subject of the question/s being posted. (Sublime1008 in no way indicated s/he didn't want to take care of Turtle, just that s/he wanted to improve the habitat).
Sublime1008,
a. I can't say it's impossible to put a canister in a 20-gallon tank; there are different size canisters. I'll speak for the Filstar, which is the canister I do have experience with. You can get an XP1, XP2, or XP3 (which is what I have). The first two are for smaller tanks, are less powerful, cost less. You could probably get away with using one. BUT, you want the most for your money, and you don't want to have to keep upgrading. I had my turtle in a 50-gallon Rubbermaid and thought the XP3 might be too powerful (wrong). The smallest tank I've had it in was a 40-gallon, and currently it's working in a 110-gallon just fine. A 20-gallon and an XP3? I don't know what kind of current it would make (when I had it in the 40-gallon I used the spray bar rather than the spigot, it comes with both). For a small turtle, I'd say no--I wouldn't want him whipping around the tank. I would think with the 20-gallon a smaller canister would be better, but then there's upgrading when you get that bigger tank...Also remember, no filter is going to keep a tank completely clean--those canisters are made for fish, not turtles (but you knew this, I think).
b. The bigger tank would be prettier, yes, but it would also make life easier for you in terms of cleaning as well as healthier for your turtle.
You're decision, but I think if I knew I was going to get that bigger tank (really) soon, I'd spring for something like the XP3. Have you looked at
www.bigalsonline.com ? You could compare brands/models there.
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-