as you feed the fish,corals, etc the POOP also feeds the corals. But some settles every where and get disolved into the sandbed,rocks etc.(as well as the uneaten food) This makes the nutrient level go up as well as other things. You have to manage the amount you feed and how often. 1st sign green HA, brown algae on glass. In there lies the problem, you have to be able to export(remove) the excess nutrients via FUGE(mangroves,chaeto,etc) and/or Carbon and GFO or both in reactors. Initial water for make up water MUST be GREAT meaning purified. What you start with is what is the building blocks of the tank. Good water,Good rock, and just understanding your tank as it matures. EVERY tank is different and one thing that works for one doesnt always work for the other. When you find what works for your set-up you will have found the MAGIC, all you have to do then is keep it that away.