The silicone that you see in the tank, isn't necessary. the silicone inbetween the glass panes, that is what holds the tank together, thats what bonds the glass, not the bead thats on the outside of the glass. I have a 240g tank, I have no bead of silicone running along the inside of the tank glass, its all inbetween the panes. Just get rid of that bead of silicone, you can use GE I Silicone or Aquarium Silicone if you feel better about running another bead along the inside. But trust me, if the bond breaks inbetween the panes, that bead isn't going to hold anything together.
To clean the tank, you can use vinegar and water, for the tougher spots you'll need to let the vinegar sit on it for a bit.
Or use muriatic acid for a quick clean up.