You might get bacteria bloom in your tank,water gets cloudy for couple of days,but then clears up.Some people reports cyano outbreaks but I never heard of algae bloom caused by biopellets. Understand that biopellets still have a lot of unknown,some people swear by them and some other have lots of problems.Every system is different and different thinks will work.If I were you I would get to the bottom of the problem first and find out what is the cause of your nitrates being high. Is your bioload to heavy for your tank,do you feed to much? Try to correct this first.If you want to start pellets make sure you go slow.Start with third of recommended dosage,monitor your nitrates for couple of weeks.If they don't go down add another third and do the same for couple of weeks.If your nitrates start to go down do not add anymore until they get to zero (don't add anymore pellets) or stop at some level,then add another third of recommended dose.This should do it.I hope this make sense for you.