I just ordered a pocket microscope I should receive it today 60-100x it was $10. I am not sure if it is worth it but I could not pass it up for the price. I want to be able to look at my refuge critters close up. I will let you know how it works.
You should be able to count 15+ in a small area when looking through the loupe to verify your density. My high density cultures will have hundreds of rotifers in a 5ml area. For the most part once you get a dense culture you will know...the phyto is exhausted quickly and you can see them sparkling everywhere.