A water conditioner is used to remove chemicals such as chlorine and chloramine from your tap water. The chemicals that water companies put into the water to remove bacteria are harmful to fish so you must use a water conditioner every time you carry out a water change. Most water conditioners neutralise chlorine and chloramine on contact so you can add the water conditioner to your tank and then top it up with tap water after a water change. If you have got a large aquarium and therefore carry out large water changes you would be wise to shop around for water conditioners and find the best value for money. Some water conditioners don't really go over a long way, for instance you may have to use 25 mL for every 10 gallons of water, obviously this means that a 200 mL bottle is not going to last very long if you are changing 100 gallons of water. I use a water conditioner called which goes a long way, you only have to use 5 mL to treat 50 gallons of water. 250 mL of Prime will treat something like 1000 US gallons of water. So for someone like me who changes 100 gallons of water every week, a 250 mL bottle of that costs £11.95 will last me a few weeks.