you might be right, biopellets can reduce o2 levels. but boipellet reactors, especially the cheap one are very tweeky and if you aren't careful can also strip nitrates and phosphates. I mean like less than 0.0005ppm in a matter of days or even hours causing a total tank crash because fish, coral and coroline alge need these nutrients to live, just is small doses. whats known as Strippage can be best prevented by buying a good reactor like a reef dynamics one where flow rate through the reactor is controlled. the second thing I might suggest is that you might be using a poor quality pellet. cheaper pellets are corn based and come with a whole lot of problems, including o2 reductions. try swapping to the better fermented pellets like the ones from reef dynamics or dr tims