Hey, I had that problem too, I think it's some sort of heat loving bacteria or fungus. You should head to the local pet store and get a pill of Eryomycin or something like that, it won't effect water or tubes and will prevent/kill all the creepy-crawlies...
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XP 1600+ AGOIA-Y (GreenLabel, UNLOCKED!@/ NF7-S 2.0/ PC 2700 DDR
10.5*172FSB(1806/ 1991 high), Vcore 1.792...@84.95Watts
--86' Chevette Heater Core
--1/2" Vinyl tubes
--Via1300-Stealth Mod(QUIET!) 
---Home made copper-tube block
---PVC small internal res
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