Let's get some things straight here:
There is no such thing as an animal that 'cleans the tank'. I read a good book on this. It was called Everybody Poops
On a more serious note, anything you introduce the the tank to 'clean it' will do nothing more than suck up detritus or scrape algae off the glass and deposit it on the bottom in the form of poop. This is espescially the case for Mystery Snails, who happen to be champs at this sport. I would go so far as to say that a single mystery snail will contribute more waste towards your tank than a betta could ever hope to.
If you want a small 'cleaner' to eat detritus and algae (which effectively saves you from scraping the glass lol) I have a few to recommend who, pound for pound are not very big waste producers:
-Shrimp. Don't ask me where they put it all but they are artists at the act of reducing algae on a low bioload-budget. I suggest ghost shrimp for bettas becase they are the least likely to get eaten.
-Nerite / Zebra snails. I would recommend them to anyone. Hands down the best algae eaters and they have very low waste output. They will only breed in brackish water so they won't overrun your tank. I would only suggest 1 or 2 for a 5g because once they run out of algae it is sometimes hard to tempt them to eat lettuce. They also won't eat leftover food. Fortunately betta pellets allow for very few leftovers.
Plants are also a good bet for soaking up dissolved waste... did you know that duckweed is used to purify water in sewage treatment plants?