Honestly, it is more trouble than the tank is really worth. You must remove the entire inner seal, which means you need to remove the top rim (if present) and it is going to break, and will not be easy to replace. The tank needs to be thoroughly cleaned of all silicone residue, (razor blades, steel wool, MEK or Acetone and a lot of elbow grease) till the glass is squeaky clean (anything less is unacceptable.) All this needs to be done without damaging the structural seams (between the glass panels) either physically or chemically. As i said, more trouble than the tank is worth.
Old axiom is 'if it ain't broke, don't fix it.' If it does not leak, don't mess with it, because it just might leak when you are done...unless the inner seals are just thrashed...and then I would admonish you for buying someone else's problems....
never a deal or good value...
As far as silicone goes, nothing is made for aquariums, and nothing can be considered as unsafe, unless it does not cure for a week or more. That said, at least use a quality silicone (not GE I) like Dow corning 735, or 999A (Graniger.com i think, at least they used to have it...) don't know how easy it would be to get in black...the high end stuff you read about, (hopefully) will skin over before you get half way through the seal placement so I would not recommend it. I use a pneumatic caulking gun and I am fast; it can turn into a big gooey mess in a little over five minutes or so...
