Aquarium sealants. Silicone made the all-glass modern tank possible. Both Dow-Corning Chemical and GE have long made silicone sealants suitable for aquarium use. Dow's brand is DAP. In 2000, GE repackaged its Stock #012 clear silicone "Household Sealant" (in the familiar blue and white tube) as #012A, relabeling it "Window and Door" and disclaiming responsibility by labeling it "not for aquarium use," doubtless because they can't control how we clueless spazzes use the product. Fair enough, in this litigious society. The repackaging caused ripples among aquarists, but RJ reported his discussion with a GE consumer relations representative in an excellent Jan 2001 post to the killietalk newsgroup. The gist of it: it's the same old sealant. There are still some lingering fears about possible long-term low-grade toxicity from possible anti-mildew agents.
Toxicity? Reservations about whether a silicone sealant is non-toxic should be clarified: the silicone elastomer itself is inert and non-toxic. Silicone sealants for certain non-aquarium uses have anti-mildew additives (arsenic, etc.) that are designed to release long-term low-grade toxicity. Read the packaging. Conversely, you can be absolutely sure, when you see a premium-price aquarium sealant packaged as "for aquarium use," that it's only being marketed — and not custom-manufactured — for aquarium hobbyists.
For a secure bond, use alcohol or a non-oily solvent to clean the surfaces free of dust and lint, soap residue (you shouldn't be using soaps to clean a tank anyway), detergents, or oil and grease (including what may be left by a fingerprint). The surface must be dry.
Curing. As the silicone elastomer cures, acetic acid is formed by contact with atmospheric moisture. A curing time of 48 hours is minimal. There should be no hint of vinegary acetic acid smell. Though silicone has bonded after 48 hours, it continues to gain strength. Optimum strength hasn't developed until about a week has elapsed.
Long after they have cured, silicone sealants continue to absorb water and swell. Make allowance for expansion when you're sealing rocks together, for example, so that the sealant doesn't start to show later.
If you're planning on repairing a leaky tank, begin with Leighton Irwin's article "Fixing a leaking aquarium" at FishChannel.