While you can put fish in purified water, it's generally not recommended without proper preparation and supplementation.
Here's why:
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Lack of Essential Minerals: Purified water, by its nature, has had most of its minerals removed. These minerals (like calcium, magnesium, and potassium) are crucial for a fish's osmoregulation (maintaining proper fluid balance), bone development, and overall health.
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pH Imbalance: The purification process can sometimes alter the pH of the water, making it unsuitable for fish. Most freshwater fish thrive in a pH range of 6.5 to 7.5.
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Lack of Buffering Capacity: Minerals in tap water (or properly treated water) help buffer the pH, preventing rapid and harmful swings. Purified water lacks this buffering capacity, making it more susceptible to pH fluctuations.
What to do if you want to use purified water:
- Remineralize: The most important step is to add a water conditioner specifically designed for aquariums. These conditioners replenish essential minerals and electrolytes.
- Check and Adjust pH: Use a test kit to check the pH of the water after adding the conditioner. Adjust if necessary using pH adjusters (available at pet stores) very carefully, following the product instructions precisely. Small adjustments are key to avoid shocking the fish.
- Regular Water Changes: Even with added minerals, regular partial water changes (e.g., 25% weekly) are important to prevent the buildup of nitrates and other harmful substances.
Why Tap Water is Often Preferred (with Treatment):
Tap water, after being treated with a dechlorinator, is often a better option than purified water because it naturally contains minerals. Dechlorinators remove harmful chlorine and chloramine, making the water safe for fish.
In summary:
Purified water can be used for fish tanks if you carefully remineralize it and adjust the pH. However, it's often easier and potentially safer to use dechlorinated tap water. Always test the water parameters before introducing fish.
