Link to Giant (and smaller Globes) as in the following image. The water looks murky (my bad photography) but it was crystal clear). Balanced and only ONE betta in it. I used the old fashioned Dr. Innes method of aquarium set up (adding plants slowly and later fish slowly to allow the bio "bugs" to adjust and gwow).
Link below to a product called bio digesters. (does not replace a tad of taking water off the bottom and replacing with fresh, like when it rains. I would use heated, ph adjusted, filtered water - NYC water was nasty - I kept in large garbage tubs and just dribble it in from the top - no rash adjustments for the occupants) but keeps the aquarium water crystal clear.
Called biodigesters for ponds but the person with whom I spoke sold me a smaller amount (think someone on one of the older aquarium sites bought it in bulk and sold it in smaller quantities). Link below. http://www.complast.com/BALLS/GLOBES/index.htm
BioDigesters - “The Natural Alternative to Chemicals” http://www.biodigesters.net/
Put a small pinch (teeny) in cup of same aquarium water - let sit then put into filter. Use as needed (each water partial so on)... a teeny bit goes a long long way. You will see little bugs hopping, swimming, zooming about in a few days - harmless and beneficial bio cultures!