thats what i was thinking.razor blade gets right down to the glass and chops up the junkFor my used tanks, I use a razor blade paint scraper (made my mainstays for $2.00 at Walmart)(assuming they are glass), and scrape staying 2-3 inches away from the silicone, then attempt to use elbow grease on the edges. It makes them shiny and new....looking.
You got three 3 foot tanks!! i'm so jealous. I wish i had the space and the tanks.. ANyways I would use some vinegar or just a good scrubbing brush and some elbow grease.