I would strongly recommend against using anything other than a weldon type product for acrylic seams in a display tank, let alone a large display tank. There are different types of weldon, #16 is thick, thicker than syrup and comes in a squeeze tube, where #4 is like gasoline and is applied with a needle applicator.
Other products will/can cause crazing and could cause failure, weldon is not expensive and I would not risk using something like epoxy that could have an unknown chemical reaction.
If the tank needs to be "re sealed" the only way to do so would be to cut every panel apart with a router or saw (router with a flush bearing is best)184, prep the surfaces and re-glue with weldon. Typically acrylic tanks are never re-sealed as the bonds are chemical and the level of effort to do this is non trivial.
Also the good thing about acrylic is that the seams are crystal clear, if there are any issues with the seams you should be able to visually identify them.
If you can post a picture of the trouble areas it should be easy to tell if its repairable, otherwise adding a bead of weldon will do little if anything to help out structurally. The chemical bond of acrylic to acrylic is much stronger than the adhesive bond between glass and silicone.
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