I used ONE of the
API Root tabs in my 55 gallon and it turned the tank a little darker than normal. I also used ONE in my 10 gallon planted
betta tank and it didn't (although the gravel is much deeper so it got buried all the way).
I'm assuming that since it goes in the tank, it's safe for the fish, so it's probably not a big deal that it's turned the water brown. There are some liquid fertilizers, so basicly it just turned into that instead of being in the substrate.
I don't have a ton of experience with fertilizers/live plants so someone correct me if i'm wrong...