most common corrosion inhibitor: dipotassium phosphate
if you're really concerned about algae: copper sulphate (can be purchased commercially as aquarium treatment solutions and there's a pool/spa mix as well). Low concentration!!! (1ppm)
bobkoure, no light helps greatly
Bill, agree on the distilled water rant, but that alone isn't a solution... ask someone that deals with water cooling klystron tubes and the like.