Aw, I'm sorry. Let's talk about catfish then.
In the 20 gallon you should probably stick to something that does not produce a huge amount of waste.
Everybody likes corys! They do not eat algae, but they do eat leftovers while they're fresh. Also like catfish tablets, and love live foods. They need to be in schools of at least 3 individuals (of the same kind). They are very peaceful, sociable creatures, great fun to watch. They need somewhere to hide, like a cave or a piece of wood. Common hardy corys are bronze corys (including albinos) and peppered corys, but there are many more on the market. The panda corys have a reputation for being sensitive. Bronzes and peppereds can grow up to 3 inches, most other kinds stay at about 2 inches.
Otos are good algae cleaners, but can be sensitive when first acquired, should only go in a mature tank. Make sure the ones you buy have a full tummy and dont look thin.
For the 40 gal you might want to look at a larger school of the above or a small pleco or ancistrus.
The bristlenose plec (ancistrus) is peaceful and hardy. Eats algae and greens, but also fish food. Males have the most amazing bristles over their noses.
There are small species of plecs out there, such as the clown plec (said to be quite shy) and quite a few others. Make sure you do not buy a common plec or a sailfin though, as neither can fit in a 40 gallon.