I picked up a Water Sprite and an Otocinclus, today. To say that the Otocinclus started eating diatoms immediately would be an understatement! He spent zero time learning his environment, he sank onto a plant and set to work. I better go get some algae tabs! Not the prettiest fish, they had some expensive panda loaches that were beautiful, but I already like this guy's work ethic.
The mechanical filter medium in my HOB was also pretty sparse and didn't seem effective, so I bought some Fluval foam pads and stuck it in the filter with the other one.