ie run both bio pellets and have recently changed over to zeo on one of my systems.. i can tell you this and its only opinion and have no scientific back up for this except what ive observed. i ran bio pellets in my system for a while.. and started to notice after abt 6 months that my corals started becoming very dull, and little to no PE, but my water parameters were good except for po4, which just would not come down.i started getting algea on my sand bed corals were either RTNind or STNing with out reason, so I finally decided to ditch the bio pellets. the pellets being a carbon source and after a while i belive the system starts o become a mono bacterial system and it lets bad bacteria form and flourish which i believe becomes the root cause for the rtn events. once i took the bio pellets out and replaced wth gfo, and then started a vodka vinigar dosing regimine to maintaine the carbon source the water cleared up within a week and the corals looked 100x better.. i did not put the full zeo on this system but rather, i would dose the zeo back 3 drops 2x a week, with continued use of the vodka vinigar as my food source(carbon source) i also started dosing zeo amino acids daily 2 drops, also ive made sure that my stronium levels are kept up as well as my potassium. i dose lugols once a week 2-3 drops.. and add my daily supliment of 2 part with bi weekly water changes.. i have totaly turned my system around in abt 2 months.. and the PE and new growth isn incredible.. So im a firm believer that bio pellets by them self are not good.. but when used in conjunction with other supliments they will do ok.. but i think that single carbon sourse with out the constant addition of diverse bacteria will ultimatly case a mono bacterial system.. with a diverse system you get a better nitrate/phosphate breakdown bc the bactera is competing for food, when it becomes mono you will only lower one type of element...
sorry for the long reply.. hope it helps