Whether you bought a tank that has been sitting in a garage off of Craigslist, dealing with a pest algae, or recycling a culture setup, at some point in this hobby you will find yourself in the position of having to sterilize something.
One of the best ways to do this is with the use of Chlorine. To start, fill a bucket with fresh water that will accommodate the object you want to sterilize. If you are sterilizing an entire aquarium, you can use the tank as the bucket for the purposes of this article. Add chlorine to the water, (not the other way around), until the water takes on a noticeable "swimming pool smell". The room should be well ventilated if you cannot go outside. Allow the object to soak in the chlorinated water for 24 hours. Aeration can be applied after 12 hours if you are worried the water will foul overnight b/c of chlorine dissipation. However, if you do that you should add more chlorine as it is likely to dissipate faster once agitation is present. After 24 hours if the water still has no discernible chlorine smell you should add more chlorine to the bucket, and wait another 12-14 hours. Chlorine breaks down as it reacts with organic compounds, so if it is hard to tell the presence of chlorine after the initial wait there may not have been enough added initially to react with all the organics present, and more should be added and more time given. Alternatively, measuring for a chlorine reading over 10ppm will work if you have a chlorine test. I usually save chlorine tests to ensure the object is later chlorine free since chlorine is fairly cheap and it easy to tell if there is a sufficient level of chlorine in a bucket. Laziness may also be a factor.
After the object has been exposed to sufficient chlorine levels for over 24 hours you can be pretty confident the object has been sterilized. The next step is to remove the chlorine so it is safe to use in an aquarium setting. It is best to remove the chlorine from the container first before draining it, so it is safe water again and not a chlorine spill in your lawn or wherever it is drained. This can be done with heavy aeration over a period of time, the use of sulfur dioxide, or a number of commercial brand name de-chlorinator products, such as "Prime". When using a product to remove chlorine, aeration will still help to speed up the process. Use five times what is recommended on the package, it won't hurt anything to do so in a sterilization situation, and you will want to be sure that the water is chlorine free. These products are also relatively inexpensive, and the recommendations are usually adjusted to deal with the amount of chlorine present in tap water, not a sterilization situation. When chlorine readings in the bucket have dropped to zero, drain the container and rinse it and the object in fresh water. Now refill the container or aquarium with fresh water, add the object to the water again, and add chlorine remover again. This time you can follow the package recommendations on the chlorine removal product. Wait 24 hours, if the water in the container shows no reading of chlorine the object or tank is sterilized, and is ready to use. The second go around is time consuming, but ensures there is no residual chlorinated water inside the object.