Hello; I will attempt torepost portions of two posts that I put on this forum earlier. The used postswill follow.
If a tank leaks, I find it best to reseal all the corners at the same time. (Inmy experience the silicone needs to be applied in all corners while it is stilltacky so that it will bond to itself at the apex of each side.) I also cut outthe existing silicone with a utility knife from all corners before trying toseal with new stuff. (If the first application of new silicone does not sealthe leak, I cut out all the silicone that I have just applied and seal thewhole thing again.) I also find that the silicone tubes that fit in a caulkinggun to be the most handy way to apply silicone. The tubes of silicone that yousqueeze with your fingers are just too hard for me to control and I have nottried to use them in years. You can plug the end of a big tube of silicone witha nail or something round to keep it from curing too quickly, as you may needit again. (Note – When you cut the end of a silicone tube, make the first cutnear the end so as to have a small opening. You can always cut off a little bitmore if the bead of silicone is too small.) I had to seal a tank last summerthat had sat empty for a lot of years, it took three tries.
Hello; It looks as you are going to attempt to seal a different tank. Some morepointers in addition to the ones in a previous post. I usually give a tank agood cleaning before applying the new silicone. I use a paste made fromordinary table salt (Without iodine) and a soft sponge. (Do not use asponge or cloth that has been used with soap or chemicals.)(I have buckets andsponges that are used only for my tanks.)The salt paste does not scratch glass(be sure the sponge or cloth used to apply the paste with is not coarse enoughto scratch the glass on its own and watch for bits of gravel that seem to jumpinto the salt paste.) and does a fair job at removing the deposits from thesurface of the glass. (Letting the salt paste dry to a haze should desiccatemost critters if you want to disinfect a diseased tank.)(Note- wetting theglass will soften up the deposits somewhat.) (Also for stubborn mineraldeposits, a razor blade in a holder that gives a firm grip is useful. Use ashallow the angle on the blade and keep an eye on the corners of the blade. Thecorners sometimes get bent and will scratch a thin line. Rinse the salt pasteout and let the tank dry well.
Once the tank is clean, I think it needs to be completely dry before applying abead of new silicone. Dampness tends to hide in the very small seams and in thecorners of a tank. I suspect that dampness prevents the silicone from bondingproperly 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 sitfor a day once the silicone has been applied, so you do not have to move ituntil the stuff can cure some. I usually try to wait for two days before addingwater to test it.) If you manage to lay down a decent bead of silicone try toresist poking at it in an attempt to smooth out a bump or irregular spot. Iusually try to smooth out the glob where the corners meet at an apex. You mayget 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 justenough to dampen the tip of the finger and not enough to drip off.)(Does itsound like fun yet?). You will want several paper towels or cloths handy tokeep the silicone wiped from your fingers. It will get on everything if youdon’t and is a real pain to clean off. If you use a cloth to wipe silicone the stuffdoes not come out, so do not use mom’s favorite towel
I think you posted that the plastic frame is loose on the bottom. Not had thatparticular issue myself, but would likely try to silicone that in place and letit cure a couple of days before attempting the recaulk on the inside of thetank. Other on this forum may have pointers they are willing to share with you.
If you happen to mess up and need to clean up a smear,( and find that the moreyou wipe it, the more it smears.) it may be easier to let the silicone set upso that it is no longer gooey. It can then be rubbed off with a cloth or yourfingers.