There is a large variety of other aquarium items and accessories available for aquarists to choose from, but only some are absolutely necessary.
You will want to have enough air line tubing to run between the air pump and the air stones, as well as some extra air stones. Over time, air stones begin to clog, which reduces their efficiency and causes unnecessary wear on the air pump.
You should also have an extra set of replacement diaphragms on hand for the air pump. If not, your dealer can replace them for you when they wear out.
Some hobbyists like to be prepared for larger problems. If you are willing to make the investment, a back-up pump and even an extra heater or filter provide insurance against equipment failure.
A gang valve is very useful for distributing and regulating air flow. Air from the pump goes first to the gang valve and then separately to each air stone. The air stones can then be regulated individually, and if the pump puts out more air than you need, excess air can be routed through an unused valve.
Routing the surplus air through an air stone reduces any noise coming from this line. Bleeding off excess air keeps the diaphragms in the pump from wearing out too quickly.
If the air pump is going to sit on the floor or on a shelf that is below the waterline in the tank, a one-way check valve should be used in the line between the pump and the gang valve. Should the electricity go off and the diaphragms stop in the wrong position, water could be siphoned from the tank through the air lines into the pump, ruining it.
A check valve will prevent the water from reaching the pump. If the pump has more than one outlet, there should be a check valve for each line.
You can use several different items for keeping the inside of the front glass free from algae. A sponge or plastic pad on a long handle, a pair of magnets -- one with a cleaning pad and the other with a felt pad -- or a long-handled plastic scraper will make it easy to remove algae.
A good-quality, one-step water conditioner should always be available for regular water changes as well as any emergencies, such as having to set up a hospital tank for a sick fish.
A supply of mechanical filtering material and granular activated carbon should be on hand for regular maintenance. If the filter uses special seals or O-rings, an extra set of these should be kept available.
Buy at least two nets. It is easier to catch fish in the tank by using one net to guide or force the fish toward the second net, and it's also a good deal less stressful for the fish.
One of the best investments you can make is to purchase a hydro-vacuum to help with tank maintenance. These gravel cleaners are inexpensive, but they're essential tools for keeping the tank clean and healthy.
Finally, you will need a bucket or two, paper towels, and glass cleaner. There are many other accessories that you can buy, but it is better to get the tank up and running for a while first. Then you will be able to determine which, if any, of these other items you would find useful.