I got a little 2.5 gallon tank free, but any small, clear tank you're not using would work.
Clean and dry the tank thoroughly.
Flip the tank upside down, and draw lines 1 inch (or 1 cm, or some other unit of measurement of your choice) apart with a sharpie across the bottom of the tank. Let it dry a few minutes.
Choose some leftover paint (I used plain old acrylic wall paint leftover from a room in my house), and paint the bottom of the tank. Pick a color that is lighter than your sharpie so it is easy to see the lines. Paint the entire bottom of your tank with 1-2 coats to cover it fully and let it dry.
Now you've got a handy container you can use to observe your fish and monitor their growth.
I use mine:
-When buying a new fish. After acclimating them to the quarantine tank water by floating their bag, fill the mini tank with water from the q-tank, add PraziPro or salt, and give the fish a short dip to try to shake loose any parasites that may be on them in that before adding them to the q-tank. While you're waiting, snap a bunch of pictures and you'll always have a record of what size and condition they were upon arrival to compare to later.
-For observing fish who may be ill. Put some tank water in, gently move the fish into the measuring tank, and you can observe them closely without interference from other fish or objects in the main tank.
-For moving fish. If I'm setting up a new tank or transferring a fish to a new aquarium, I can mix water from the old and new tank in this one, add the fish, and carry them to their new home with minimal stress.
-Every month or two to take a reference photo. Once you start to get a collection of these, it is an easy way to compare how fish are doing, and learn about how differences in your tank set-ups, feeding, or the genetics of siblings affect their growth and color as they age.
-For some medical treatments. This is a nice size if you have the misfortune of needing to sedate a fish for a procedure, give a dip in something like Tricide Neo, etc.
The bowl in action!
On the top you can see Slim (the smaller one) is 2 inches long* in a photo from the day of her arrival on 3/10/07:
In the most recent reference photos (7/29/07), you can compare, and see Slim is 4 inches long and developing a girlish figure:
(*measured nose to peduncle)