Bottomfeeders:
Eat fresh food left over before it rots
Do best in groups
Have large appetites
I suggest cory catfish, they're community fish, and do really cute things, and although they eat food off the bottom, and stores say that they're bottom dwellers, they swim through all three levels all throughout the day!
Algae eaters:
Eat algae off glass, plants, and other surfaces
One male maximum, multiple females are fine
Watch out! Large bio-load!
Are naturally grazers
Do not get a common pleco, they need 100g+ tanks to live in!
I suggest bristlenose plecos, that have a minimum tank size of 30g and they are very cute. They come in many strains, so keep an eye out, because some strains might need a larger tank than you have! I hope for the best! ^^