I've run my sponge filters with and withouit airstones. You can actually see the increased flow with an airstone attached. You can test the difference in flow for yourself just as I did. Some of the better-quality sponge filters do include attachments for an airstone.
Another advantage of the smaller bubbles produced by an airstone is a quieter filter with less surface disturbance.
Whoops. Sorry, CP3. Here's the one I mean.
[ame=http://www.amazon.com/Jardin-Aquarium-Cylinder-Sponge-Filter/dp/B005LMQCW2/ref=pd_sim_petsupplies_4]Amazon.com: Jardin Fish Aquarium Mini Cylinder Soft Sponge Water Filter, Black: Pet Supplies[/ame]
I've never seen one in person, bit it looks to have a top-fitting plate similar to an ATI Hydro, which does take an airstone.
I would not follow the recommendation in the product description to replace the sponge in six months to maintain the bio filter. That is just the opposite of what you should do.
Following that recommendation will kill the BB and force a cycle to occur.
I have never seen any objective evidence that would indicate that sponge filters operate more efficiently with an airstone and until I do I intend just use the sponge filter input designed into the filter. I also wonder why the mfr wouldn't design a place for the airstone if the airstone was an improvement. They could then sell an airstone with every sponge filter. Or build the airstone into the filter in the first place, they would then get more revenue for the filters.
Oh yea, def not going to replace the sponge! I always find it silly they suggest that.
I almost got that one, but I wasn't sold on the outtake sticking sideways like that. I read a review where someone mentioned it created a flow across the tank that annoyed their betta. Not sure it its removable or not.
There are several experienced keepers right on this forum who are using that filter or similar ones, CP. That double-barrel job is really big, way overkill for a 5g. They make single cylinder versions which would be more appropriate.
[ame="http://www.amazon.com/Aquarium-Biochemical-Sponge-Filter-XY-2821/dp/B009V3UGDS/ref=pd_sim_petsupplies_10"]Amazon.com: Aquarium Biochemical Sponge Filter Fish Tank Air Pump XY-2821: Pet Supplies[/ame]
I still think that Jardin Mini is a better deal. It's nice to be able to pick up the filter to vacuum around.
Sugarunicorn, I think the design logic behind that 90* elbow on top is to release the water flow without bubbles disturbing the surface. It may also help to muffle the bubbles. After all, it cannot take an airstone. It seems that water level would be critical.The smaller diameter riser tube makes the water flow faster, hence complaints by Betta keepers of too strong a flow. Most of these products are designed to fully disassemble in order to clean them thoroughly.
I think you'd be better off with this one.
[ame=http://www.amazon.com/gp/product/B007VCHSKU/ref=oh_details_o02_s00_i00?ie=UTF8&psc=1]Amazon.com: Como Fish Tank 6 Layer Sponge Biochemical Water Corner Filter Black: Pet Supplies[/ame]
It's a better size for a 5g. Also, shoving as wedge-shaped filter into a corner cuts off a lot of area that could and should be used for filtration and the bacteria colony. If done right, two filters can be run off one airpump.
Question -- if you're running two filters off one airpump (i'm assuming with a 3 way control valve), would you want to get a stronger pump? Or is the 10gal tetra whisper sufficient?
The Tetra Whisper 10 airpump is plenty for two sponge filters. You could use a 2-way valve or, more cheaply, a T-fitting and 2 cheap air adjusters.
I might mention here: beyond a certain minimum, the size of the sponge makes no difference in the size of the bacteria colony. The bacteria in your tank will only be just enough to handle the ammonia converting needs. No more is needed; no more will grow.