I dosed vodka for awhile. The best stuff to use is whatever is cheapest. I have now switched to dosing sugar since it is waaay cheaper, adjusted for the amount of carbon you are adding. I dosed about 30cc in my 500gal system daily. Most just dump it in. Since I had an extra dosing pump I wasn't using I dosed it 24/7 with that.
What I notice is that the skimmer produces more and that it is tan colored instead of dark green. My filter sock quickly get coated with bacterial slime and I have to clean it almost daily. If you add too much you will see strands of bacterial slime coming off the rocks. That is a definite sign to cut back.
I figure that by constant dosing I get a more even bacterial growth that gets removed rather than a bloom-crash cycle. So far I have seen no ill effects on corals and I have seen a die-back in problem algae. Interestingly, although the over-all amount of algae has decreased, I also have small patches of different kinds of algae that I hadn't seen in the tank before. Likely the nutrient shifts have favored them over the turf algae I had before.
Allen