That depends upon its chemical make up. Some minerals found in spring water could make the water unfit for aquatic life. My advice is to do a complete water test. In some fish, the water has to be the right consistency and temperature for the fish to survive longer, I suggest asking an expert at a pet or aquatic shop for the best results.
Some people do not believe it but the best water to use in your betta fish tank is tap water. Bottled and distilled water has been "purified" and does not have the minerals that your Betta fish needs. If your tap water is so bad that you do not even drink it, than use spring water.
Salt Water Fish
Assuming the dirty fish tank has had fish and water in it, without doing various specialised water tests it is impossible to tell what is in it.
It might be unsafe to drink because fish food, fish scales, fish waste, and many different kinds of bacteria are in fish tank water.
Live Aquaria provides you with supplies to preserve a fish tank with salt water fish. Fish tank provides lighting, fish food, and more to help you maintain your salt water fish tank.
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The white scale on the inside of the fish tank is from a buildup of minerals that were in the water.
Robert Frost
Scoop all your fishes to another tank or tub of water and clean the fish tank just as you would normally without the fish
the cause might be a sick fish, a dirty tank, or over feeding.
put the live fish in a bag filled with water and change the tank then put the fish back in.
you can fish the fish out of the tank with a net or a cup and keep them in a bowl with some water that came from the unclean tank, until the next tank is ready for their arrival. So when you have the next tank ready, you can just take the cup and dump them in with the water that you left it in when cleaning.