I am a bit controversial on this still. Ive heard both sides but I had a bad experience with 100% silicone that had a mildew inhibitor. I used this silicone to put in the baffles to the sump of one of my aquariums. I let it cure for about 3 days, got it going and then a couple of days into it it started to get like a greenish/yellow film on it. I wiped off the film and it continued to form for a couple of weeks, maybe even a month or two. This aquarium was going to be my quarantine for both corals and fish. Everything was fine in the aquarium as far as water parameters but my fish would die out of the blue. The $1k+ I spent in corals also died for no reason. I did nothing different in this tank than in my other 3 tanks except the use of this silicone. I tore it down, redid the baffles with GE I and I got none of that film, the silicone is still nice and clear, and the livestock is doing fine. I cant guarantee that that was the exact problem, but its kind of a coincidence that with different silicone I didn't have that problem.
This is what I used:
This is what I used:
