Canister filters only have brand new carbon (since Friday). I just changed 50% of the water. Nitrate reads 20 PPM. It is back on the charts but it was 20PPM before the water change. I have checked every nook and cranny of the tank, it is clean including the sand bed. Do you have any experience with the seastorm (see image)? That is the only place I would imagine that could be the problem at this point.