How do u change freshwater tank in to brackish water tank?

do use table salt if yes how much ( 30 gallon tank want to buy some 8 figure puffers )
Answers:
Although Petco will tell you that most of their fish are freshwater fish, many tropical fish and most sharks need "brackish" water. Brackish water occurs in nature when a freshwater system such as a river flows into a saltwater system. (Think the Rio Grande river flowing into the Gulf of Mexico, for instance.) Try picking up some aquarium salt at your local pet store, or you can add a bit of *NON-IODIZED* table salt. Most fish can adapt to a little salt, so do not worry if your "brackish" fish have tank mates. And as long as you follow the directions on the package (one level tablespoon per 5 gal. water) your filter will work just fine. You won't notice a difference, but your fish sure will, and they will love you for it.

Adding salt is beneficial in improving gill functions, maintaining proper organ qualities and functions, and can improve healing if your fish become sick. Even fish marked as "freshwater" can still adapt and benefit from brackish water. Many times, freshwater and saltwater fish can both co-exist in brackish water, since it is a combination of the two. Both types of fish will adapt to their environment.

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by adding brackish water additives, you can get this at your local pet store. just specify the type of fish you have in your tank and how many they are and how much water you got and how old your fish are,,, if any is a boy or girl you must factor that also

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Adding salt, but not table salt! Brackish is the area where freshwater from rivers mixes with the sea. You'd be better off adding a synthetic sea salt mix for your puffers.

They won't need a lot of salt, so just get a small package of the mix. You can get a hydrometer, too, and mix your salt so the hydrometer reads 1.005-1.008. Puffers are very sensitive to ammonia and nitrite, so it would be a good idea to cycle your tank before you get the puffer - he'll be happier and healthier! Here's info on how to do this: http://www.fishlore.com/nitrogencycle.ht...
http://www.firsttankguide.net/cycle.php...

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Don't use table salt.

It contains iodine which is bad for fish.

You need to get pickling salt or aquarium salt.

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Add 1-2 teaspoons of salt per gallon of water.

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I had an avocado puffer and i just added a pinch of rock salt. He was healthy!

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Add a Sea salt mix. How much all depends on what type you're using. It's easiest to prepare your water and add a half cup of salt at a time until get to the salinity that you want. (I use 5 gallon buckets, use a smaller measure if you use a smaller container) Once you figure out how much salt you use for your container, it's easier the next time. (A hydrometer is a must)

If you don't have any fish or plants in the tank, and it's up and running already, you can add the salt directly to the tank a little at a time untill it gets to the correct salinity. (Let it sit a few hours between additions to let it disolve completely). Or mix a large amount of salt into a small amount of water and let it disolve there, then add it to the tank.

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You do not use table salt. But special aquarium salt and add it to the right degree. Then you can add in the fish.