Depends how much you put in at once and if the LR is well cured. (One way to know it ISNT is if it smells really bad in spots.) Also how much "stuff" is on it that will likely die off? I've been adding pieces here from the lfs to the main tank and there and no spikes. I now have a 20 gal set up to ensure the rock is "cured" before I put it in my main tank.