Whether your aquarium suffered an irreparable crack and you want to recycle it or you simply want to use the glass for a do-it-yourself project, taking apart a glass aquarium, particularly a large one, requires preparation and care. You'll need to take precautions not to hurt yourself, so gather up some gloves and safety goggles, and remember to be gentle with the following procedures so that you don't do any (further) damage to the glass. And, obviously, make sure the tank has been fully emptied and dried before proceeding.
Put on your protective goggles and gloves.
Place the tank over a spread out towel (or several towels depending on the size of the tank). This is to safely capture any glass fragments that may chip off during the process.
Remove the trim. The trim is the plastic around the top and bottom of the aquarium and can be removed by softly hitting it with a mallet, loosening it. You'll then need to wiggle it with your hands until it comes loose and can be removed. Be careful when striking it as to not break the glass.
Scrape the silicone off the corners of the tank using the razor blade or razor scraper. This silicone adhesive holds the five pieces of glass together. Remove as much of it as you can from around the edges of the tank. The glass pieces should still hold together, however, as there's still silicone connecting them (where the individual pieces of glass meet). That will be removed next.
Apply heat using the heat gun to a corner of the aquarium. This will soften the silicone holding the glass sheets together.
Cut in between the two intersecting pieces of glass in the corner using the fishing line. You should be able to saw through the thin layer of silicone between the panes of glass.
Repeat steps 5 and 6 with all four corners of the tank and where the bottom glass sheet meets the four wall sheets. If done correctly, you should be able to separate all five panes of glass from each other.
Remove any remaining traces of silicone using the rough sponge.
This method is best for square or rectangular tanks, although odd-shapes such as wave tanks or hexagon tanks can be taken apart in the same fashion (though hexagonal tanks will require more time since more panes of glass are used).