As tap waters most common disinfectant element is chloramine, which is in itself very harmful to fish when taken in through the gills and to pond life in general, a counteracting agent is required to neutralize the chloramine element within water you are using to fill your aquarium.
Not doing this correctly may result in disaster. Fish gasping above the water surface, erratic behavior and burning/scarring around the gills are a telltale sign something is not right - even if you keep your aquarium well aerated. The counteracting agent chemical is commonly referred to as a "chloramine conditioner".
Another factor to bear in mind is heavy metals - the kind of heavy metal elements that build up in your pipes - especially metals such as copper, lead and iron. Where as we all need a trace element of heavy metals in our biology - it will be too much for your fish to take. Before filling your aquarium be sure to run your tap for around 2-3 minutes to make sure any dislodged heavy metal particles are clear form the water flow. A good idea is to run the water into a bucket, then use that water to water your plants so as not to waste any.
There are three types of water conditioner available:
Of course, you should not buy any of these products before understanding the quality of tapwater you have available to you. You can either request this information from your local water supplier, or you can use water test strips. We recommend using king British "6 in 1" water test strips.
We also recommend the following water conditioning products to help get the best possible water quality for your aquarium.