For your tank I recommend:
Shrimp, may it be ghost shrimp or amano shrimp are very easy to care for, eat left over food and in the case of the amano they eat also algae if your tank has any as well. And do not have much bioload and you can buy them in groups of three or more so they feel secure. The only risk is that they may become a meal for your betta.
Snails have a higher bioload, most people have apple snails and mystery snails, but in my case I picked the nerite snail, which doesn't grow too much and unlike people believe there aren't hermaphrodites, which means that one day you won't wake up and see your tank full of snails. And even if you do buy a female and male, they cannot reproduce without brackish waters.
Assassin snail, it's a small snail that eats other snails o.o great for parasitical snail pests and interesting to watch, their shells are also very pretty and unique, the one downside is that you need thin sand substrate because they like to burrow.
A bit harder are Otocinculus, or otos for short, they are mainly algae eaters, and have a high death rate when you first bring them home, if they survive for a month in your tank, then that oto is ready to endure anything, they are very sensitive to water changes, that's why. The down side of this cute creature is that if you do not have any algae in the tank you may have to supplement algae wafers, and your betta might nibble on them. Also they need to be in groups of 3 so they can feel secure.
Neon tetras: they are schooling fish and relatively easy to care for, they need to be quarintined well, because they are very prone to suffering ICH. And you need to buy at least 4-5 so they can feel safe, just make sure that the ones you are buying are healthy
Cory cats: make sure they are small, they eat wafers and are bottom dwellers opposite to the betta. There are many types, panda, bronze... you can google them to see for yourself.
Zebra fish (or Danios, olympia told me the name heheehe)they are also a type of schooling fish.
And White cloud mountain Minnows which are also schooling fish. One of my favorite because they are so interesting to watch and very very very Hardy XD
Schooling fish, require +4 groups and considering tanks I would limit myself to 6.
Keep in mind: If you get schooling fish, you might not be able to get anything else.
if you want to have a good amount of mates, and different kinds then +20g may be needed.
ALL TANKMATES SHOULD BE QUARINTINED BEFORE BEING ADDED TO THE TANK FOR AT LEAST 2 WEEKS. To make sure they aren't carrying anything like parasites from the petstore. __________________
Last edited by asukabetta; 12-28-2012 at 12:40 PM.