So I shouldn't have used this stuff?
Well, this is a bone of contention among hobbyists, so this isn't an easy yes or no question. Personally, I doubt that whatever makes the silicone mildew resistant isn't water soluble. If it was, you would think the mildew resistance of these silicone would be washed away by rain.
The more important thing is that there are better silicones altogether for putting together a tank.