The problem with ultra low nutrient aquariums is just that, they have very low nutrient concentrations. While this filtration scheme, which is often accomplished through dosing various chemical additives, is great at reducing nitrate and phosphate, and by extension unwanted algae growth, it also puts a strain on coral health. Corals of all shapes and sizes take up nitrate and phosphate to some extent, as they feeds the photosynthetic algae that live in their tissues. By depriving coral of these essential nutrients, hobbyists are essentially starving the corals in the long term. This situation often leads to poor coral coloration, a lack of polyp extension, and sometimes even bleaching events, especially because the corals have a heightened sensitivity to water parameter swings. Because of this, reef keepers have to dose coral foods such as amino acids back into the tank to get the desired coral coloration and improve coral health.