I know this isn't recommended in the original guidelines post, but is it possible to keep one, two, or maybe even 3 AFDs in a 55 gallon non-aggressive tank? I read somewhere else that the only problem might be the food not sinking to the bottom, but to try and use a turkey baster to drop it directly to the bottom for them.
Also, any clash between them and a pleco, either him eating them or them bothering him?