A lot of shops sell "glass worms," which are white mosquito larvae. They come in frozen packs just like bloodworms, and I must say they GO CRAZY over them. I left mine out overnight last night, though, so off I go to buy more. -_-
Just found an old bucket with dirty, aged water and lots of algae and got a ton of mosquito larvae. I let them sit in the freezer for an hour to cull them and now there outside. My WCMM's love them, not sure about bettas yet.
All u need is a 5 gallon bucket fill with water
3/4 to the top
Place some dead grass in it
One or two stick that long enough to touch the bottom of the bucket n stick out a foot of the bucket
Put the bucket in a shaded area(all day)
Like a under a big tree with a lot of small plants around
Keep the surround area moist
One to two weeks top
You'll have hundreds of little larvae
Do a few buckets
So you can control the size of the larvae
When I was living in Hawaii, I left a large plastic container filled with small pebbles, tank water and algae out on the back porch of the house, and within a short amount of time, had enough mosquito larvae to feed all of the bettas. Inexplicably, within a few months, I also began to have tadpoles appearing in the same container. The baby frogs were too cute, but they were taking all of the mosquito larvae for themselves.
I'd harvest by scooping some larvae into a little jar (whilst being careful to avoid catching tadpoles and frogs) and using a pipette for feedings. Just be aware that if you don't keep a close eye on the development of the larvae in the container out back, you may have a mosquito infestation on your hands sooner or later.