If you are getting leaks from a reseal, it isn't the silicone that is the problem. You need to look at your procedure. I would suspect that you are leaving trace amounts of the old silicone. There are stronger silicones such as GE 1200 series (possibly the RTV also, but I'm not sure). This would be desirable in the building of a large tank. The 1200 series is meant for contractor use and isn't normally seen in stores that don't service the trades.However, a reseal doesn't require the strength of a build as all it does is provide a seal without adding to the structural integrity. Any silicone should work for that. The one thing I always do, once I am positive I have removed all the old silicone is to get a new blade and go over it all again, sometimes twice. After that, a good cleaning with acetone or alcohol to remove any traces of skin oils or other residue. It is imperative that you do not touch the area to be siliconed with your hands. A pair of nitrile gloves my be a good idea to avoid any risk. In addition I like to extend the new silicone beyond the old bead in order to contact virgin glass. This isn't always possible if the previous bead was sloppily applied.
One thing that is never mentioned is application technique. You want to lay the silicone in with no voids in it. This means careful application of the bead and forcing it into the vee with no air inclusion. This sounds simple but does require some consideration, as you don't want to "drop" it in. If you are new to caulking or not really comfortable with it, (this tip from another forum) you can practice on a cardboard box with any cheap caulk to get the feel for it. The other thing is that an overly large bead is of no benefit and is more difficult smooth cleanly, which is important to ensure good contact on the edges of the fillet. if the edges are not clean and tight there is the risk of water starting to get under the silicone at those points.
I surmise you are not a total beginner when it comes to this, so examine your technique. The one thing that would concern me is that you mention the tank leaks from different spots. It sounds like the initial build was sloppy. A well assembled tank will hold water without the inner seal.
Regardless, don't give up. Almost all my tanks came to me as leakers and I am still using one I built in the mid 60s.
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