What are you feeding the rotifers? You should be able to see them if you look very closely/under a magnifying glass. The best way to know they're doing alright is that the green tint goes away after feeding them.
A glass tank works best because you'll need to look through the tank to "measure" how green the water is. This gives you an indication of how much and how often to feed phyto plankton
I used a salt bucket with near three gallons of tank water, a heater and an air stone. I added Nannochloropsis algea daily and harvested every two days. If you put some of your rotifer water in a clear glass and hold it to the light you should be able to see them with your naked eye (provided you have good vision).
If you take a look here you might find some more insight on how to grow them