Ugh, that looks like a nasty. I think the only way to salvage this tank without making it look like a complete mess would be to remove the top piece of glass if you can. Silicone comes off fairly easy with a razor. The tricky part would be removing it from the frame without damaging the frame. Replace with window screening or if you need it to be really sturdy use perforated stainless steel (more expensive and commonly found at custom auto shops, I use this as I have cats who like to sit on my tanks).

I have had success in stopping cracks like this from spreading by smearing silicone along the crack on both sides. I've done this for glass on cold frames out in the garden and they've held together since. It looks really ugly though, and is no guarantee against further breakage. You could also get a piece of glass cut, smear it with silicone and apply the whole thing to the top of your tank like a big bandaid. This would be even uglier, but sturdier.

Was your light sitting directly on the tank or was there some airflow between it and the glass? What wattage was it?