Unfortunately, this may be another prime example of the illusionary power of The Matrix. Tn a sort of "law of mass conservation" way these vermin will excrete a fair amount of toxic waste, and will remove almost as much precious Oxygen from the water as occurs during the decay of uneaten food. The more food they eat the more they grow and the more waste they produce, the more Oxygen they consume. And because they move at a snails pace, some food still remains uneaten and rots. The only way to improve the "cleaning" coverage is to have lots of them crawling over your tank, which defeats the purpose. I suppose, some species are prized for there reputed removal of algae from hardscape, but these are often huge, garish and unsightly. The better policy, naturally, is to just not get algae, which renders them redundant.
Infestation can be reduced significantly by application of Super-Mega-Ultra-Nova-High CO2 concentration levels........which.....of course, may not be a suitable option if there are fish present in the tank. And of course, this treatment may not have much of an impact on eggs, so execution of repeated carpet bombing missions via modulation of your knurled needle valve knob is necessary. Be sure to remove the debris. Many will seek refuge by escaping via the tank wall.
CO2 is the gaseous version of duct tape. It's usefulness and application are innumerable....
Feed your fish predominantly live foods, which can be done at a significantly reduced interval (owing to it's higher energy), to mitigate the increased cost, and live food will never remain uneaten.
Cheers,