Planted Tank Enthusiast
Join Date: Sep 2010
Location: Southern California (LA)
Even though most people will tell you to use the larger filter...you don't necessarily need one. A 2213 would be fine for a 17.5G (a 2215 would work too) and a 2213 could even ben fine with a 33G.
It all depends on what you want to do and if you have future plans for upgrading.
Since your tank will be low tech, you won't need a lot of flow in there to disperse CO2 or ferts so you could get away with just a 2213 in a 33G. I learned recently that many people on TPT use a 2213 on even larger low-tech tanks.
If, however, you hope to inject CO2 in the future or would want to upgrade, I would invest in a larger filter. This way, if you do inject CO2/ferts/etc, you won't need powerheads which you'd need with your 2213.
If you can figure it into your budget, I'd go with a 2215 since you're deciding between a 17G and 33G. Its not too much more than 2213. The jump from 2215 to 2217, however, is big. A 2217 would be too much for a 17G low tech and is not worth the investment for a 33G low tech.