My previous Instructable, a gumball machine fish tank, came out nicely, but had a few problems. One was the need for it to be on a stand, and another was how unstable I felt the bowl was on the base. I reworked my design and it feels much more solid now.

You will need:
Gumball machine: I prefer round, glass globes, but it's up to you.
1/4" thick glass, cut to size, with polished edges
Aquarium sealant. Regular caulk may contain chemicals harmful to fish! Don't use it.
Betta light
Small tank filter
Gravel
Plant (optional)
Colored glass marbles (optional)
Assorted threaded rods and nuts
Foam (for noise dampening
Extension cord
Rubber grommets (assorted sizes)
Rust-proofing paint
Felt pads
A fish

Tools:
Drill
Jigsaw (or a lot of patience with the hacksaw)
Hacksaw
Sandpaper
Assorted screwdrivers
Razor blade (for trimming dried sealant)
Assorted wrenches
Masking tape

Step 1: Disassemble and Clean

Simple enough; take the whole machine apart and clean it.

Step 2: Paint Inside of Lid

One of my mistakes on the first iteration was not painting the inside of the steel lid with a rust-proof paint from the start. This resulted in water splashing and rusting the bare metal, then dripping down and poisoning the fish. I painted the inside of the lid and the retaining ring with rust-o-leum white rustproof paint after all the drilling and cutting.

Step 3: Seal the Bowl

The ~6-liter bowl has a 5 inch diameter hole at either end. Last time I sealed it with a single 5" glass disc on the outside of the bottom. I was never happy with this, as I worried that the silicone could one day crack and the bottom would fall off. It also elevated the bowl another 1/4" above the base, giving it a look that I felt was awkward. I remedied this by having a 1/4" thick by 5" diameter glass disc cut, and then sliced down the middle, giving me two half-circles. I could then lower these INSIDE the bowl, and seal them together on the inside.

Step 4: Attachment

In a normal gumball machine, two rods run from the retaining ring on top of the globe, down through the container, and screw into the base. Since there will be glass in the way, the globe must be attached differently. In my previous attempt, I fashioned a hold-down out of scrap aluminum which I siliconed onto the bottom of the bowl, and secured down by turning a threaded rod running through the bottom. This is problematic twofold. First, it isn't fantastically secure. Second, it requires that the whole thing be on a stand, since the rod needs clearance.

This time, I instead ran two threaded rods though the back of the machine, through the hopper, which was secured with silicone to the bottom of the tank. It simplifies removing the tank (just pull the two rods out) and sidesteps the need for a stand. It's also much more secure.

Step 5: Internal Components

Inside the base of the gumball machine, I housed the power, air pump for the filter, and wiring for the light. I didn't really change much this time, except for using rubber grommets from the get-go, and a jigsaw to get a clean cut for the power cord. Again, I wrapped the air pump in foam to keep it from rattling against the metal base. I also added felt pads to the bottom so it wouldn't scratch whatever surface it was on. A 6" threaded rod is to secure the cover to the base.

Step 6: Cover

Once the cover dried, I was able to silicone the light to the inside, and attach the retaining ring and lid together.

Step 7: Water Testing

48 hours later, the silicone is cured and ready for a water test. It passed on the first try.

Step 8: Base

Run a bead of silicone around the bottom of the bowl, and then put the hopper on, making sure that it's lined up so you can get the rods in. I let it dry on the base, so it would fit perfectly.

Step 9: Finish Up

After everything is dried, it's pretty straightforward. Run the airline and light wire out the back of the base and up to the cover. Gravel, plant, insert filter. Plug it in and you're good to go. The colored marbles I ordered hadn't arrived yet, but I put them in later, along with a betta. It's much quieter, since the air pump has much more padding. It looks cleaner, too, since the bowl sits 1/4" lower in the base, and the cover fits a bit better.