Planted Tank Guru
Join Date: Sep 2004
Location: Garden City, KS
I doubt you have to worry about heat from the light affecting the bond of the silicone to the glass... unless you're just not sure that you bonded it correctly, in which case there are many threads available in the search function that discuss that technique.
But presuming you bonded the glass properly, I'm sure heat won't be a problem.
...That being said, what may be a problem, on the other hand, is the weight of the light fixture. Without seeing your actual work, it's hard to say, but be sure that the two extra panes of glass you bonded on are PERFECTLY attached, and aren't off-axis. Even a couple degree tilt could be disastrous if that fixture is heavy enough.