If the 15 gallon will do the trick though there is no need, so I'll measure it up first and see what I have. A quick guesstimate though says I will need room for about 5.5 gallons of back flow (that's 2" drop in the tank, which I doubt it will be that much), and there would be about 6 gallons room in the sump. That is pretty close, but I'm pretty sure that I won't have 5.5 gallons back flow.
The only places here that sell Aqueon (that I know of) are Petsmart and Petland. Petsmart doesn't do any special orders at all so if it isn't a regular stock item they can't get it. Petland has outrageous prices so I refuse to shop there unless it is dire need.
I will go check Petsmart this weekend though because I think I
might have seen 20L there.