I'm afraid limestone chips won't work terribly well because the pH is above 7.0. They'll work a bit but not as much as they would if the pH was below 7.0.
@Sgooosh
General hardness, GH, is a measure of the amount of calcium in the water. If there's a lot of calcium, it's called hard water; if there's not much it's called soft water.
Hardness - GH - is very important for fish.
When fish have evolved in soft water - water with not much calcium - their bodies hang on to most of the calcium in the water. If they are put in hard water - which has lots of calcium - their bodies still hang on to it but because there's so much in the water it builds up in their bodies and deposits form in their kidneys. They do not live as long as they should.
When fish have evolved in hard water - water with lots of calcium - their bodies remove most of the calcium they take in from the water. If they are put in soft water - which has very little calcium - they still continue removing the calcium. They can't get enough calcium from the water so there is not enough in their bodies to keep them healthy and they get sick more easily.
Carbonate hardness, KH, is a measure of the amount of buffer in the water. Buffers are things which react with acids to stop the pH falling. Carbonate has no effect on fish and we don't really need to know what the KH of the water is. It is not important to the majority of fish keepers. The only time it is important is when it is very low and then the fish keeper needs to make sure that their pH doesn't suddenly drop very low.
The fish don't care what the KH is.
The Rift Lakes in Africa are different from most fresh water. They have very hard water with high KH and pH. When fish keepers want to keep fish from these lakes, they have to keep them in very hard water. Some people have suitable tap water, but many don't. If the tap water is not suitable, the water has to be changed. This is done by adding Rift Lake salts to the tap water. This is stuff which contains chemicals to make the water hard, to add KH to the water and to raise the pH. These slats can also be used to make the water hard for other hard water fish. But other hard water fish don't need it quite as hard as Rift Lake cichlids so the fish keeper has to test for GH to make sure they've added the right amount for their fish.
Can you look at your water provider's website, please. Tell us what number they give for 'hardness'. If they also give something called alkalinity, can you tell us what that number is as well, please.
If two tanks are filled with the same water they should have the same GH, KH and pH. Unless we do something to deliberately change these parameters they should remain constant. Adding limestone or coral will increase all three a bit. The effect is greater if the pH is below 7.0. Adding lots of wood or peat can lower them a bit.
You say you had an old tank which had harder water than your current one. Are you talking about GH, which is what everyone else means by hardness, or KH, which only you seem to think is hardness?
If you do mean GH, there are some reasons why the current tank could have softer water than the old one.
#1 Was the old tank in the same home as the current one? If you have moved between having the two tanks, the place you live now may have softer water.
#2 Your water provider could have changed where they get the water from.
#3 The old tank could have been suffering from lack of water changes. A number of fish keepers don't bother doing water changes. All they do is fill the tank back up when the water level drops through evaporation. When water evaporates, the only thing which leaves the tank is water. Everything dissolved in the water stays behind. When the tank is topped up, more water containing dissolved stuff is added, so the tank now has more dissolved stuff than before. The dissolved stuff includes calcium. So every time a tank is topped up, there is more and more calcium in the water so the hardness increases.
If regular large water changes are done, everything dissolved in the water is also removed and the new water just replaces the dissolves stuff, it doesn't add more to it.
You mentioned brackish water -
Brackish water is in between fresh water and sea water. The stuff we usually call salt, the stuff in the kitchen, is sodium chloride. That's just one type of salt, there are hundreds more. The salt in sea water and brackish water is sea salt - a mixture of sodium chloride plus a lot of other salts. Adding common salt to water doesn't make it brackish, you need to add special sea or marine salt for that.