Well, you wanna have them for a couple of reasons... if you have a mandarin or any other fish that relies on those little crustaceans, they will have something to eat. They also eat off of micro-algae including diatoms, etc, etc.
It's a good thing. 
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30g reef tank, CPR CY192 filter w/ DIY plumb running Rio 17HF return, Coralife 3x 9w UV Sterilizer, 3 24W T5-Helios 10K Daylights/3 24W T5-Helios Blue lights, 2 Logysis blue meteor light strobes (moonlights/24 Blue LEDs).
Tiger tail cuke, asst. snails/hermits, asst. feathers, rainbow acan,zoos,shrooms,bubble,galaxea, asst. shrimp, 2 ocellaris,mandarin,zebrasoma xanthurum,pink-spot watchman,red-striped pistol. | 
copepods? 
I installed a 4bulb tek5 setup yesterday. Much to my amazement when I came home from lunch today, to study the life in my tank as I always do, I saw a swarm of tiny white little crawlers on my glass. Well being new to saltwater my first reaction was "#%$@ there's some kind of infestation in my tank, what next!! I tried to do some research on the net but I must be using a bad search query. The only thing that I can come up with is that they are copepods? Is this right? 
jb
