What's the brand of the zooplankton? 20 bucks?
Like bro fuel suggested, dip and shake the zoo in freshwater for several minutes. MOST of the Nudi will fall out. However, some will still survive the dip. Dip again after a few days when the zoo recover from the shock. Try to catch as many nudi you can see.
Also, zoo need plenty of light to stay open. Try putting them at the highest level. Will have better chances to keep them growing.
Disclaimer: Above are based on personal experience. Will take no responsibility on lost of livestock.
Btw i can't help noticing your signature..