well, if its left too long under water theres a chance it can leach phosphate, and it nigh on impossible to tell when its exhausted, it dosent last as long as most posters say it does, id suggest 2-4 weeks but it depends on the water quality in the individual tank, also it varies from varieties in quality, its difficult to know with accuracy which brand is best, although some tests have been carried out here on RC, also it can shock corals if used in high quantities for the first time as it clears the water so much the extra light suddenly being illiminated to the corals sends them into shock, well this is not my findings but ive read this a number of times, also if not washed or rinsed out properly it can leave a film of gunk on the water surface, however in everything ive mentioned the pros definately beat the cons
