Tap water is available almost anyone keeping an aquarium, and is almost always going to be fine for your fish. If you are starting a freshwater aquarium, tap water will be perfectly fine unless you are certain the tap water in your area is of horrible quality. Still, you do need to be aware that if your tap water is from a municipal water supply, or if you have a holding tank for your water supply, you probably have chlorinated water, and you will need to use a dechlorinator or a chloramine remover to make the water safe for your aquarium, depending on which compound is used in your water system.
Some of the people might think rainwater is clean when they live far away from the cities. Rainwater is always lacking in necessary trace elements to keep the fish healthy, as well as those necessary to help the water maintain chemical balance. The truth is that the pollution can affect a much more significant area than you think. It is not a good or not safe source of water at all for aquarium fish.
The bottled drinking water is another choice for fish aquarium. Bottled water is often not regulated or tested, so the water parameters are unknown. Also, bottled water often has vitamins, preservatives, colors, flavors, or other additives added to make the water more appealing. These can be devastating to your fish tank. It will cost a lot more, and you will have to pay attention to the ingredient on what is in the bottled water. Some of them might not be just H2O, and they can have additives which can be deadly to the fish.
Reverse Osmosis (RO) Water for Fish
Another option for your fish tank is RO water. The processes of creating RO water got clear of all the substances, which make it 100% pure H2O. You can get RO water by getting an RO unit. Some of the fish stores also sell RO water for reasonably low prices.RO water is pure, which means it has absolutely nothing else in it also H2O. It can be potentially dangerous in an aquarium where the slightest change can cause the PH to crash due to no dissolved minerals is acting as a buffer.
Important: If you decide to use RO water for aquarium, you must add buffer back into the water before using it for an aquarium. It can do easily by using commercially available aquarium water buffer products. The advantage of using RO water is that it is clean with nothing harmful in it for the fish. Most importantly, you can choose your water hardness by adding a different amount of water buffer. It is option to create an ideal environment for your specific fish species.