When you started keeping fish you probably didn't have a need for a holding tank. But when you have to break up a fight between two aggressive fish or when one becomes ill, the holding tank reveals itself as invaluable. A holding tank doesn't need to be fancy or decorative, but you must take some factors into consideration when you're setting up a homemade holding system.
Huge Isn't Necessary
A homemade holding tank is a temporary solution for a fish health or behavioral problem, so the tank doesn't have to be huge. In fact, according to Maddy Hargrove and Mic Hargrove in their 2010 book "Freshwater Aquariums for Dummies," a small holding tank is beneficial. You can more readily observe your fish's condition in a small tank, which is easier to clean, too. One gallon of water per inch of fish is a general rule of thumb for holding tank capacity.
You don't need to go all-out decorating your homemade holding tank, but you don't want it to be barren, either. Your quarantined fish needs a few floating and rooted items to make him feel secure and keep his stress levels down. "Few" is the key word. In a smaller tank, there won't be room or the need for a large collection of plastic and live plants and other decor. A couple each of stationary and floating items are all your fish needs. Minimal decorations will allow you to keep an eye on him.
Your homemade holding system can be made from a glass or even plastic tank. Just make sure that the interior walls are smooth and that the tank doesn't leak. Your holding tank will need a removable lid that attaches securely to the top. It will be more convenient if the lid has at least one hinged hatch. Your holding tank will need a filtration system -- but don't use carbon with it, as carbon will filter any medication you give your fish out of the water. You'll also need to provide agitation to the surface water with an air stone or a power head, and have a heater to maintain water temperature. Keep a spare test kit on hand to ensure that pH, nitrite, nitrate and ammonia levels are appropriate and constant. Keep an extra fish net for netting fish who are ill.
If you keep different types of fish, you should have a holding tank for each unique type. For example, you wouldn't want to put a saltwater fish into a holding tank prepared for freshwater fish, or vice versa. Many people overlook the need to quarantine corals, but you should have a holding tank especially for corals if you keep them in your aquarium. That doesn't necessarily mean you'll have to have a roomful of holding tanks, but you should have at least one set up and ready to go, and two would be better. Use water from your main tank to fill the holding tank when you need to use it. If you have a saltwater tank, you should keep a supply of saltwater mixed and ready to use since you cannot prepare saltwater and use it right away without harming your fish.