First things first: If you're going to turn a jar into an aquarium, you can't skimp on standard aquarium supplies and maintenance. You'll need a pump, filter, thermometer, gravel and clean water. You may want to include some greenery and underwater structures, such as rocks and reefs, as well. And depending on available light, you may want to add an aquarium light. And of course, for a project like this, you'll need a really large jar -- a standard jelly jar just won't do.
The time to create a jar aquarium is before you have fish to put in it. You'll need to set up the aquarium and let it stabilize for a couple of days before introducing the fish [source: Petco]. Before you combine fish in an aquarium, learn which species don't do well together. Learn about swimming preferences and territories, as well as appropriate foods.
Remember that appropriate aquarium size depends on many factors, including the type, number and size of fish. Very few jars will be large enough to handle multiple fish. However, a small jar aquarium demands more maintenance, because less water means less stability. It's easier for a contaminant or a pH imbalance to affect all of a small amount of water, whereas in a large tank, the large amount of water can help neutralize problems [source: Domagalski].
All that said, it's possible to maintain an aquarium without necessarily raising fish. A row of glass jars containing corals and seaweed could be a lovely and inexpensive way to add color and serenity to a room. Planted aquariums often require stronger lights than other aquariums do, but you'll be rewarded with vibrant greenery [source: Domagalski].
No doubt about it: it can be difficult to have aquatic life depend on you for all its needs. For a somewhat less complicated way to reuse your old jars, take a look at the lantern project on the next page.