i've heard both. i've heard people say they've used the gauge for years with no alge buildup but some say they've had problems with it. if you think about it though, fungles may be able to grow inside the measuring tube, but once they get to the inside of your res, which has no light in it i would hope, they would die, so i don't see it becoming a problem unless it unstabalizes your PH in the long. you should be in the clear enough to leave it like that, but if problems do come of it be sure to let the city know.