I like what RayJay has to say. Everyone has different methods, you just have to find one that works for you.
+1 about them crashing when one doesn't clean out the container. That is my experience also.
If you put 1million rotifers in a
5 gallon bucket you'll start with around 60 rotifers/ml. Somewhere between 60 and 200 per ml seem to be where my cultures run. If you have a syringe that can do a ml or a part of a ml; take a half a ml or so and squirt it on a reflective surface (I use the screen of my tablet), put it under light, you can count the rotifers. Keep a log of it, maybe take some photos.
I think adding more algae would be a good idea if you are having issues and the water is clear often. Make sure you change out a consistent 20-30% of the culture water daily (8 or 9 pints) with aged newly mixed C2O. But give it maybe 3 days in the
5gal bucket to grow before you start doing your
WC's or harvesting. So on day 3 or four, start harvesting by taking 8 or 9 pints out, and pouring them through the sieve, discarding that water, and adding aged newly mixed saltwater to the culture. Be sure to change out buckets once a week also, leaving behind any goo on the bottom.
In serious commercial rotifer production they have them crazy dense like they are talking about. To them 500/ml is probably low density,
idk.
Also, temp doesn't matter that much, but at 78 the culture will be at pretty much peak production, and use up algae the fastest, but grow the fastest.
You could try dropping your SG down to 1.015 (in addition to the increased algae feedings). I'm not sure what your issue is, but they do grow better at 15 than 19 I think.