Several ways to reduce the hardness:

1. Diluting your hard water with some type of soft water. That is RO water, rainwater, deionized water. If using rain water, do not collect right away, since the first part of the rain will be full of pollutants. RO water can be expensive.

2. Filter your water over peat moss or peat plates. This will vary depending on the grade of peat and will also stain your water to a tea like brown

3. Filter thru commercially available water softening resins.

If you soften your water and the hardness keeps coming back, check your aquarium for any sources of calcium, such as sea shells, the gravel, decorations, etc.

Thanks for your suggestions Dear Tepe. Are there any chemicals which reduce hardness of water. For example CaCl2 increase the water hardness.  

You may try zeolite for your purpose. Here is a paper regarding natural zeolite (clinoptilolite).

cordially,

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I found some information about Zeolite that can be used as filter. I think, we cannot mix it with water. 

Ca(OH)2 (or CaO) and Na2CO3  (lime and soda) are typical chemicals used for reagent water softening. Sediments with insoluble product are formed as a result of reaction.

I tried na2co3 but hardness was increased When drop na2co3 into the water

For what purpose you are trying to soften the water?

You can use NaOH (Sodium Hydroxide), it is very effective in softening of water, although it is bit costly but it is very effective.

Thanks for your suggestion. I used water purifier then adjusted with CaCL2

For a somewhat related RG discussion, yet on brines, see: https://www.researchgate.net/post/Anyone-can-advise-me-on-how-can-we-reduce-the-High-concentration-of-Magnesium-Calcium-and-sodium-from-highly-saline-brine