However the remains of a couple 20-gallon and 10-gallon and even tanks have come in handy as tank tops. If one has hanging lights, the tops will still let most of the light (excepting a little UV) in.

Somewhere around here, there are even a couple of impromptu tank tops made from the window glass in a shopping center which was broken during a demonstration during the Nixon years. Old recycling habits die hard. Boy can you pile a lot of "stuff" on top of that glass!

In one case, I took a panel from a 2.5 gallon tank and siliconed it over a crack in a 5 gallon glass tank. That is not something I would attempt with a larger aquarium. It is uglier than sin, but gives a fry tank new life. A bunch of Najas or Val planted in front of the patch and placement of the aquarium fairly well masks the ugliness.

Some manufacturers "sand" the edges of the tank panels. Others only take the sharp edge off of the outsides, so handle them carefully. I don't know how attractive it would be, but duct tape could be run over the edges to remove any risk of getting cut. There are at least three colors of duct tape (gray, black and either blue or red) around here somewhere.

Disassembly tools include leather work gloves, a box or roofing knife (the one I use is hooked), an open garbage can and eye protection. A long sleeved sweatshirt and even an old baseball hat are not bad ideas. You will either look like a dork or a master craftsman taking that tank apart, but you will be an intact dork or master craftsman. ;)

Probably the master craftsperson comes away unscathed.

This is more to the taste of pack rats than many people. However I have saved a fair amount of money and numerous fishy lives with these.

In the highly improbably case that you have an ancient metal frame aquarium in the basement somewhere, you could take a piece of glass to your hardware store and get it cut to size. Spend the extra buck or two to have them dull the edges. That piece, if it is an ends piece can be jammed over the old cracked one (ugly!) or replace the bad one. That is a lot of work, you'd want to find  a tube of DAP or GE aquarium silicone (it will say that on the tube - DON"T use one which does not), beg, borrow or purchase a $2 caulk gun and get the glass cut.

On the other fin, NEVER MIND fixing the old metal framed tank unless you are into antiques. The repair will cost more than buying a new 10-gallon aquarium. And that old stainless steel does rust in time. Almost our last one of that sort, rather   crumbled to death last year. The other three are currently storing aquarium gear.

Some of those old tanks have slate bottoms. (The steel frame, often thick glass and slate bottoms made those aquaria a real chore to move.) Whole, cut up or broken, those slates make great spawning surfaces for angelfish. If you have no use for them, there is an angelfish keeper somewhere who could use them.

Several years ago, a friend, who was a member of one of those woodcarving clubs, needed an old aquarium. Since he was carving the fish, the tank didn't need to be watertight. Heck, it didn't even need all of the side glasses in. It was a great deal for both of us. ;)

So some recycling uses of defunct aquaria aren't worth the effort. Others are.