Depends what kind of fish. If it's a Freshwater Fish, tap water from a faucet that has a water filter in it is good. If it's a Saltwater Fish, buy sea salt and mix in the right proportions to filtered tap water.
Mild fresh water with vegetation.
fresh water creeks and streams
The best place to find a salt water fish tank is in the Marine Depot. Not only can you find salt water fish tanks, you can also find aquarium supplies and LED lights.
if its a pond you have its coi they can live for years ans are a hardy fish
with a fork and knife
Fish in tanks eat fish food while there is a variety sold in stores reef frenzy is the best for salt water. In the wild fish each smaller fist, plants, and organisms living in the water.
otto cats are your best pick as long as your other fish are not aggresive
When you buy fish from a retailer - their water quality might be slightly different from yours. When you get the fish home, it's always best to gradually mix in some water from your own supply with that of the water in the bag the fish is sold in. Leaving it for a few minutes will acclimatise the fish to its new surroundings.
Spring or fall when the water temperature is lower. This makes it easier to acclimate the fish.
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.
These are tropical fish that like their water warm. Betta are best kept at 26 to 28 degrees Celsius.
If they are in a tank, make sure they are fed and the water is clean. To clean the tank, remove the fish and put them straight into another source of water. Remember if your fish is salt or fresh water. If the fish is in the sea, river or pond, its best to leave it. If the fish is on land near a water source, replace it in there. If it is nowhere near a water source, I have no idea how it got there. You would have to leave the fish then. Sorry if this answer is vague, because your question was as well.