It depends on the tank, really. Those that are fed very heavily and have tons of invertebrate larval production, etc. (usually older tanks), it is difficult to overskim. On much newer tanks, it is a very real possibility. Your corals will let you know quickly if you are doing too much. They will start becoming pale, despite having the same lighting, etc. Others that are more dependent on dissolved nutrients will just fail to thrive. You should also notice a decline in any smaller animals in your tank.
