Maybe but probably not .Once the bacteria get going on the nitrogen(probably the ammonium variety ) it still takes a while to see nitrate reduction, IME. Sometimes months.I'd double up the dose every week until you hit about 200ml for teh 600 gallon system; that's half of what I currently dose to 650 gallons ( 80 ml vinegar plus 40 ml of vodka which equals anther 320 ml vinegar) but I've been doing it for over 5 years and feed very heavily. Back down on the dose if you see significant amounts of bacterial mass in the tank or cloudy water from a bacterial bloom. Employ a good skimmer. I'd stick at 200ml for a month or so and move up further if necessary from there.
Some have reported a faster drop in the nitrates when using sugar but sugar has caused problems with corals in many tanks including mine even in small amounts.
Some will say 20ppm nitrate is ok for sps. That has not been my experience nor the experience of many folks keeping sps tanks,I know. I don't go over 1ppm even when dosing some nitrate. Again the nitrate level is arguable but ,imo ,it needs to be quite low but not zero particulary for certain types of sps. NSW at the surface of the reefs is around 0.2ppm.
Having said that your issue may not be the nitrate and may relate to some difficulty the corals are having due to other causes like: the alk swing., pests( flatwroms or redbugs) hypoxia etc. Once stn starts it often just keeps going even after the precipitating event is over and corrected. Fragging away dead spots a qaurter inch or so into good tissue is often the only way to save a specimen: while that works often sometimes the frag doesn't make it either.
