Not trying to be funny but; Do bettas fart? Or pee? just a fact I thought would be interesting to know...
Freshwater fish pee a lot. They are constantly absorbing water through osmosis (fish contains more sodium than the water), and have to rid themselves of it. They don't have urinary bladders, so they are pretty much peeing constantly. Fish pee is one of the two main sources of ammonia that fish excrete in to the water, the other is via respiratory exchange in the gills.
On the other hand, saltwater fish are constantly losing water due to osmosis (water contains more sodium than the fish), and typically excrete urea in a very concentrated form, similar to what birds do (the white part of bird poop is actually pee).
What kind is it? Is it made for aquarium use? If so, follow the dosing at it will be fine. If not, and its something like miracle grow, I would stay away.
(Ex, seachem flourish, tetra floura pride, API's leaf zone, etc are good, use them right in!)