Hi and welcome to the forum
Check the silicon for cream or white patches and air bubbles. These are indications the silicon has or is coming away from the glass and you will need to replace the silicon.
Touch the silicon too, if it is hard then it is bad. Good silicon will be firm but feel like rubber, it squishes a bit but is still firm.
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Small tanks are easy to repair but big tanks take more work. If the silicon is gone on the front panel of glass you can use a single sided razor blade and remove the panel. Clean all the old silicon off and then apply a thick bead of new silicon along the edges. Use a silicon designed for glass, that states (suitable for aquariums).
Add enough silicon so when you put the panel on the silicon is 1-2mm thick.
Use masking tape to hold the glass panel to the rest of the tank and use your finger to run along the inside corners to smooth out the silicon. Let it dry for 24 hours before putting the tank on its side and gluing the top support strips on. Wait another 24 hours before standing the tank back upright and gluing the cross members on.
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WARNING*** Do not to inhale the fumes when you have your head in the tank because you will pass out. ***
Once it is glued back together, let it dry for 7 days in a warm dry room (the average house is fine). Then you can set it up again. Try to do this outside just in case the tank leaks.
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Due to the age of the tank, the silicon might be breaking down on all the seams. If so it might be easier to replace the tank or send it to a tank builder and have them replace all of the silicon. However, this can cost nearly as much as buying a new tank. However, if you are a handy person, then have a go. It sounds complicated but it's just a matter of removing all the old silicon and putting new stuff in.
If the panel of glass is scratched you can replace that while you are doing the silicon. Buy a new sheet the same size and thickness and get the glass shop to bevel the edges or at least smooth them off. Then you have a nice clear scratch free sheet of glass to look through.
