Hello, it's just old. Aquariums usually need to be resealed every 10yrs. I've done it a few times & it's easy.
You need to move fish & all contents to another tank or a storage tote because it takes a couple of days(48hrs some say 24) for caulking to cure properly.
Scrape all caulking in corners & bottom on inside of tank but do not pull glass panels apart. Remove everything & clean w/acetone. Painter's Tape inside to be caulked & caulk whole tank at once.
I then dip finger in water & smooth out all caulked lines. Remove painters tape & let cure. The cleaning & caulking take a couple of hrs. The picture is a YouTube vid on how it's done. Hopefully this helps you!!!!!