One trick I learned was to measure my nutrient level by checking cheato's growth. For my tank whenever cheato stops growing and fade/die my corals bleach & when cheato grows well my corals do well. Since zoothanthelae & cheto are both algae, it helps me a lot and I havent checked PO4 or used activated carbon for a long time now. Whenever I use activated carbon the cheato would start to die and then corals bleach. But because I used to bleach many corals by depleting the nutrients, at least 50% of the bleached acros came back with different colors or stayed faded.