We don't count the densities. We eyeball it. After 10 years you kinda get a feel for it. We are also around all day so we can add more easily. For rotifers, they don't aggressively chase after them and they are so small, that you don't see much of a snick. If you look closely they kind of just open up and swallow them no real visible snick.
The copepods, especially larger ones will get them to do a visible snick.
For the copepods, you want calanoid species. Which ever you can culture will work. They can take food in the 120 to 300 plus micron range from day one. After 3 days or so they can take 250 to 500 microns.
Some of the studies have tried to count how many food items fry eat. One study found they can consume 1 to 2 food items a minute. So for a 14/10 light cycle at one food item per minute X 60 minutes X 14 hours = 840. I generally figure 1,000 copepods per day per fry. It doesn't take a mathematician to realize you will never have enough pods.
Ray's links should give you the info needed for the screen. I just use a dremel to cut away and then cover with 670 micron screening from Aquatic Eco Systems. Clear PVC glue is the best to adhere it to the PVC. I get several years out of them with constantly bleaching them to sterilize them between broods.
Dan
