hi everyone i have a problem about water change. Wish someone could help. I have a 900ltr aquarium with lot of rocks in it. i make water changes every 1 month by changing 200ltrs of water. nitrates levels seems to be stucked at 160ppm. And its impossible for me to clean gravel because i have live rocks standing on each other. Wont risk to drop one and make a mess. how should i lower nitrates level please? fish eats once a day. Only have 6 malawis in it. i have 2 canister filters to circulate water. its a 1500l/h each. Thanks