SO not knowing the difference a while back I bought two azoo sponge filters, rather than the trusted and true hydro sponge filters. thats the back story.
I bought a 25 gallon one and a 10 gallon one. the 10g is for my breeder tank, the 25g is for my 20L planted female betta sorority. right from the word go, these things sucked royal eggs. the 10g was hooked up to an air pump that came with my tetra 3i filter, the 25g hooked up to my monster tetra whisper 100 dual outlet air pump. they both moved about as much water as a squirt gun.
today for kicks i put a bigger air pump on the 10g filter, with tremendous results! problem solved! but the 25g filter still sucked a ton. no matter what air pump i put on it, single or split, made a difference. the azoo filters dont allow an airstone like the hydrosponge does, so it just sucks and doesnt get any better. well, im looking at the 10g filter moving as much air as the 25g filter and I got thinking. just for fun I completely disassembled it, and found the problem!! It has something kin to an air manifold in the base of it, kinda like a built in air stone. half of the holes for the air to pass were covered in muck, and most importantly, excess slag from the plastic casting process!
two minutes with a pocket knife this filter too, was moving water like a tidal wave :)
just a heads up... I still recommend the hydro sponge over the azoo oxygen plus filters, but if azoo is what your lfs carries its not the end of the world. I ALMOST bought two new hydro sponges, but a few minutes fiddling and the azoos work great too, if not as good as the hydrosponge :)