OK, I need to understand something. Are your LEDs mounted on boards/stars?

In any case, they need direct contact to a heat sink. You can either fasten them mechanically (screws) or via silver epoxy, or like I do where the star is put on the heat sink, and then kept down with epoxy (no epoxy between the star and the sink).

For many/most uses other than keeping your fish confined to your tank, silicone is a poor choice.