its not a leech. im thinking its a "drain fly" or "moth fly" or more adequately named "toilet fly" larvae,
either way, member of the psychodidae family. dark gray to black in color, found near/in sinks and tubs.
chances are there was one in the plumbing, laid eggs, and the eggs migrated thru the tubing, into your hydro set up. they eat ORGANIC MATTER as larvae and pupae, so you must remove them
the trick is hot water on the roots. not boiling water, but make it hot (steamy?)
rinse the shit outa everything else tho (basin/reservoir, tubing, pumps, thermometer (if you got one in the water), water heaters, etc.
all good man, just clear them out before they eat them roots. once they become flies, they lay eggs again, then youll be dealing with hundreds/thousands of them. so deal with it asap.
they vary in color a bit, but i think you get the idea:
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just be glad its not jeff goldbloom!
http://www.imdb.com/video/screenplay/vi3631351577/ <- might have to go watch that again, oooooold movie lol