I agree that this should be a sticky! Wonderful thread!
A quick question though. I ordered a few sponge filters online (they were a couple of bucks each, less than the sponges at the cheapo store, haha!) and ordered two different sizes because I wasn't sure which would be best (the measurements were vague, it was eBay after all). Now they've arrived, one kind is teeny tiny, the other is massive in comparison to my 3.9gal tanks! Which of these would you recommend for a tank of that size (which will decrease in capacity slightly with the addition of gravel and more plants/hardscape)?
Small: 2.25" x 2"
Medium: 3.5" x 4.5"
That's not including the weighted bases and tube, just the sponge measurements. If I were to use the larger one, I'd have to saw some of the tube off, and sand it down smooth for it to fit under the water surface...
I wouldn't want to not have a large enough unit in the tank, it'd defeat the purpose of having one in the first place, but wow, is the "medium" big for where it'd be going! Heh.