Make sure it is Silicone I (not II since that one has microbicides in it). Get it in the caulking gun tube, it is much easier to deal with this way.
Get smoked glass. This way the light from the fuge doesn't grow algae in the whole sump, just in the fuge. It is not much more expensive but is worth it for the benefit. This is also why all my tubing down there is black and not clear. Before I switched to black tubing I had coralline growing in the tubing.