Every reef aquarium has one, a canary. An excellent example of this is when a mixed coral reef aquarium exhibits a single coral or species with poor health while the other animals thrive and grow normally. A less obvious example may be the lightening of pigments or lack of normal polyp extension. These small indicators can guide you through diagnosing the possible cause and correcting water chemistry issues or other imbalance. I have found a strong correlation in one of my systems and low or deficient Magnesium. While I do not test Mg regularly as with alkalinity and other parameters, when the value becomes lower then 1200 ppm, Montipora species will lighten and show less polyp extension. A quick correction back to natural sea water levels, will show rapid improvement in this species. These Montipora species are one of my canaries. Zoanthid speciescan also tell a tale if you read them carefully. Normal extension and strong color is what you would want to achieve, but some times they look very stretched or possibly very flat and retracted. The stretching would normally indicate the polyps need more light and the opposite would most likely be true in the instance where they would be very flat and retracted. Using this method has assisted me greatly in finding the sweet spots for certain very sensitive species as well as avert disaster. I call this concept “reading the coral”.