Hello; I have tried to seal a small section on a leaky tank a number of times with mixed results. Often the patch did not take for various reasons. In the last few decades I have almost always done a full reseal. This seems to give better results and sort of feels like less total work in the long run as many of the patches failed before very long.
As to how much silicone it may take, my guess is one tube is likely to run short especially if the beads of silicone are thick. If you get the silicone from a store that allows hassle free returns, it may be prudent to have three or more tubes on hand and return the unused. You want to lay all of the beads before any of it has cured to insure they all bond to each other. I also will be going to a four or more day curing time in the future as has been suggested in other posts on the subject.
Here is an old post of mine about a proceedure use to seal tanks. There have been better methods posted by others, including a very helpful video not too many months ago. Try a search.
Hello; Here is a revised old post about sealing a tank with silicone. Resealing a tank can be a chore and may not work the first time, but if you get a good seal it should last for years.
I have found that trying to run a line of silicone only on a portion of a tank often to not work out very well. If a tank leaks, I find it best to reseal all the corners and sides at the same time.
-(In my experience the silicone needs to be applied in all corners while it is still tacky so that it will bond to itself at the apex of each side.)
-(New silicone does not seem to bond to old silicone very well)(Some have posted that it will stick better to the old if the old is well cleaned, I have yet to test this myself.)
I also cut out the existing silicone with a utility knife from all corners before trying to seal with new stuff. I find a shop vacuum a handy way to remove the bits of silicone. If the first application of new silicone does not seal the leak, I cut out all the silicone that I have just applied and seal the whole thing again. I also find that the silicone tubes that fit in a caulking gun to be the handy way to apply silicone. The tubes of silicone that you squeeze with your fingers are just too hard for me to control and I have not tried to use them in years. You can plug the end of a big tube of silicone with a nail or something round to keep it from curing too quickly, as you may need it again.
-(Note – When you cut the end of a silicone tube, make the first cut near the end so as to have a small opening. You can always cutoff a little bit more if the bead of silicone is too small.)
Some additional pointers. I usually give a tank a good cleaning before applying the new silicone. I use a paste made from ordinary table salt (Without iodine) and a soft sponge.
-(I do not use a sponge or cloth that has been used with soap or chemicals.)
-(I have buckets and sponges that are used only for my tanks.)
The salt paste does not scratch glass and does a fair job at removing the deposits from the surface of the glass (be sure the sponge or cloth used to apply the paste with is not coarse enough to scratch the glass on its own and watch for bits of gravel that seem to jump into the salt paste.) (Letting the salt paste dry to a haze should desiccate most critters if you want to disinfect a diseased tank.)(Note- wetting the glass will soften up the deposits somewhat.) (Also for stubborn mineral deposits, a razor blade in a holder that gives a firm grip is useful. Use a shallow the angle on the blade and keep an eye on the corners of the blade. The corners sometimes get bent and will scratch a thin line. Rinse the salt paste out and let the tank dry well.
Once the tank is clean, I think it needs to be completely dry before applying a bead of new silicone. Dampness tends to hide in the very small seams and in the corners of a tank. I suspect that dampness prevents the silicone from bonding properly to a surface and may result in a leak in a good looking seam. (note- this is not real critical, but try to have the tank in a place where it can sit for a day once the silicone has been applied, so you do not have to move it until the stuff can cure some. Silicone labels often say that it will cure enough in two days to add water. (While I have gotten away with around a two day cure with thin beads in small tanks, it seems prudent to wait a few more days to be sure. Some experienced people have suggested seven days for a full cure.) Thicker beads of silicone in large tanks will likely take the additional time to cure. Silicone will skin over and harden from the outside in. Push on a thick part of a bead with your finger to see if it feels firm and solid.
If you manage to lay down a decent bead of silicone try to resist poking at it in an attempt to smooth out a bump or irregular spot. I usually only try to smooth out the glob where the corners meet at an apex. You may get away with wetting your finger tip lightly with water and gently touch it. (Water on the outside of a joint should not cause a problem if you use just enough to dampen the tip of the finger and not enough to drip off.)(Does it sound like fun yet?). You will want several paper towels or cloths handy to keep the silicone wiped from your fingers. It will get on everything if you don’t and it is a real pain to clean off. If you use a cloth to wipe silicone the stuff does not come out, so do not use mom’s favorite towel
If you happen to mess up and need to clean up a smear,( and find that the more you wipe it, the more it smears.) it may be easier to let the silicone set up so that it is no longer gooey. It can then be rubbed off with a cloth or your fingers.
I try to test the tank in a place where a leak or having to empty the tank will not be a problem. I also allow a resealed tank to sit full of water for a few days to be sure the tank does not leak before moving it into the house. Good luck with whatever procedure you decide to use
