did you make your eggcrate go all the way to the sides of the tank? When i did mine i made the floor so that it stopped short of the sides and front so that i could fill that part in with pea gravel. Here's a picture to show you what i mean



And here it is with the pea gravel



If you made your's to all the way to sides all around and can't silicone the inside now, I'd say maybe use some black spray paint or black construction paper of some kind