I've never seen an otto eat an algae wafer.

They really like algae, diatoms and periphyton films.
If you really want to "feed" them, grow some nice algae films on some rocks, small piece of slate etc, then rotate the slate pieces in the tank.

Reg ferts, water, sunlight, current: rocks, that's all you need to make their "live food".

do this weekly at a min and daily if you really like them.

Regards,
Tom Barr