Distilled or RO (Reverse Osmosis) water is considered to be most purest and safest for drinking by humans. But when it comes to using the same for your aquariums it is not the best option. If fishes are kept in distilled water they find it hard to survive in it inspite water being purified.
Lets find out the reason behind why distilled water is deadly for fishes.
The distillation process purifies the water by removing impurities but it also takes away the minerals and vitamins that are essential for fishes to survive. There is lack of enough oxygen in distilled water which makes it difficult for the fishes to breathe. Even tough the tap water also contains very small of oxygen , around 5 ml of O2 per liter , it is still sufficient for the fishes to survive.
Normal tap water contains certain micro nutrients in the form of salt which are missing in distilled water. These salts are necessary to impart energy to the fishes and keep them active. Fishes if kept in distilled water for longer period tend to loose their color and become lethargic.
How can you make distilled water safe for fishes?
One cannot deny the fact that distilled water is the purest form of water. We can still use it for fishes by just making few additions to it. As we know that distilled water lacks some micro nutrients and minerals, if we can add these missing components externally this distilled water would become perfect for your aquarium. We just need to add salt , air filter and maintain the right temperature of the distilled for it to become suitable for fishes.