I have to recommend a school of female bettas. They do well with most non-aggressive tropical tankmates (mollies, platies, tetras, etc). I have nine in my 45gal and they're all over the place as far as their swimming height. Sometimes they chill on the bottom, other times they are cruising around mid-level. And you can find some absolutely gorgeous colors out there (I have blues, turquoises, crimsons, and a couple that are dual-colored...none of them washed out or marbled like some females can be).
Bettas are anabantids. This means they
do not use their gills to breathe. They are normally from shallow, poorly oxygenated waters and use a special organ to breathe the air from the top of the tank almost the way you or I use our lungs. Bettas are better off in shallow waters, higher pressures can actually cause lots of stress and injury. The reason yours stay on the bottom is because they are stressed from higher pressures and are very very tired.
You will notice your males that sit in still, shallow water hang out in the middle or near the top. That is a bettas "happy place". The only times my bettas stayed in tanks that large was when I was raising large numbers of fry ( who still use their gills for a small part of thier life cycle) prior to " jarring" or separating them.
Also, bettas do not "school" and even though females will tolerate each other, it does not mean go willy nilly with too many of them.
If you want a huge amount of color variation, go with Platies. They are bubbly, dopey, active fish that are more than willing to increase their numbers for free. Just keep 3-4 females for every male and you will be fine.
but good call on the otto-cats, they are very active and very good workers.
I would not recommend a male bette for any tank over 29 gallons. depth is bad for bettas. a happy betta is a shallow betta ("look at my pretty fins")....wait...that came out funny...
Just remember, just because something CAN be done, doesn't mean that in general it SHOULD be done!
