Small

The small size (up to 10 gallons) is perfect for a 5.5 gallon betta tank. It's quite small so it doesn't take up too much room. It'd be easy to hide in a planted tank or with some driftwood. The base is heavy and it stays at the bottom after the sponge fills with water. I paired it with the Nicrew Air Pump and it's working great.

Works well with the micro hygger air pump. It’s doing a great job keeping a 5.6 gal tank clear and clean.

This sponge filter is PERFECT for smaller tanks!! I have had it in a 3 gallon betta tank, and now have one in his upgraded 6 gallon. Since it's a short and stout sponge, it's easy to hide with plants and hardscape! Works great at keeping the tank clear and clean, and doesn't make much noise on top of that. Would 100% get again.

Works fine. A bit on the light side though, and needs persuasion to stay down.

Very nice set up but the sponge is too coarse for my betta. I like the set up thigh, so I’ll be buying a large, round, super soft refill sponge to put on it.

Works great in shrimp tanks

If you have tanks you definitely want this product. Ever since I found these bio sponges all of my tanks have them. I love them I will never go back to a traditional filter

I looked at measurements prior to purchase but this is much larger than what I had expected. Since it’s 1/4 cylinder and fits in corner it only works on the one side. 40 Gph will work for a 4 gallon tank and it is way too big to fit. The only reason I purchased it was to add additional oxygen to my tank temporarily while fighting a hair algae issue but an air stone won’t work with it!! So that was useless. Also there is no place for adding other filter medium. A small round one would work as well and be much less obtrusive.