Is the water cloudy on your culture? I had the same thing happen to me. I would strain the water from my reef tank. Basically my culture got contaminated with bacteria, which in turn choked the rotifers out of oxygen. It is best to use new sea water or water that has been run through UV to make sure there's nothing living in it. At this point I would recommend you sieve out the rotifers from their current water and use all new water that has the same properties as the one they are currently in (ph, salinity, tempearture). Then feed and do not harvest for at least a week to get your desity up. Start with a smaller volume of water, like a gallon and add a half gallon a day until you reach your bucket limit. I would also recommend lowering the salinity to somewhere around 1.018 as rotifers are more productive at lower salinity levels. You can go even lower to like 1.014 and they will reproduce faster, but at 1.018 they are within .007 of most people's reef salinity level so when they are fed, they do not die of shock. You can reduce their salinity about .01 daily until you reach your target. Also good idea to have dual cultures going at the same time, or store in your refrigerator about 1 liter's worth of culture heavly dosed with phyto (really green water) and replace it about once a week to keep it fresh. You can feed the one you are replacing into your reef after it has reached room temperature and then re-fill the container with a fresh liter of culture and phyto.