20 Tall Sponge Filter Sizing

[egruttum]

I plan to switch my HOB to sponge filters.  I have a 20 gal tall with 8 guppies, 1 bristlenose pleco,, MTS, Ramshorn snails, and some ghost shrimp.  Based on other pictures I would say its somewhere between light and medium planted at this point.  I purchased three small Aquarium coop sponge filters but plan on using two for this tank and the third for a future tank.  Since I am an engineer I did sizing calcs to decide on two smalls instead of one medium.  Using just the circumferential surface area two smalls have the same surface area as one medium.  With having 4 base surfaces instead of 2 the two smalls should have more surface area than one medium.  Are two smalls alright for my aquarium?  I plan on one in each back corner.  I might keep the HOB as a polishing filter but I want to be able to rely solely on the sponge filters.  Will two smalls be alright or do I need to upgrade to two mediums?

[Ryan S.]

I think you will be fine.  I use the mediums with no issue in 20s and agree that surface area should be very similar with 2 smalls.

[Garren]

Two small should be just fine. I am currently using 2 Nanos in a 20 tall with no issues. 

[egruttum]

Thanks for the replies.  I'm guessing that volume of sponge is the deciding factor for it's biological treatment capability?  But surface area is important for pressure drop through the filter (controlling flow rate) as well as velocity through the filter which controls contact time with the biology.  Anyone know the biology and kinetics behind filter sizing?