02-05-2010, 03:20 AM
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The short answer is that there is no accurate way of predicting it. Basically, plants will use ammonia and nitrate as food, but that depends on tons of other factors (the type of plants, the lighting, the amount of other nutrients present, etc.).
You can monitor how quickly your nitrates climb to get an idea of how effective your total biological filtration is (plants included). You want to keep nitrate below 20ppm (the less the better). So basically, ignoring all of the other factors for the sake of argument, you should be able to stock a tank such that doing regular water changes results in nitrate that never gets above however high you set your bar for acceptability (10ppm, 20ppm, etc.).
Shrimp have a very small bioload, so they shouldn't really affect the nitrate levels too much. Purely in terms of bioload, small fish would probably not push you over the limit either. In reality, though, you're not going to be able to find a small fish that lives in small groups or alone that will peacefully cohabitate with a betta in a tank of that size. In other words, I'd just stick to things like shrimp and snails.