Shells, limestone and dead coral rubble are all made from calcium carbonate and will help raise the pH and might help increase the KH a bit.
If you want/ need to raise the GH, then buy some Rift Lake water conditioner and add that to the new water before adding it to the tank. The Rift lake water conditioner is a mixture of different salts (chlorides) and includes things like calcium chloride and magnesium chloride, both of which raise the GH. The Rift lake conditioners also contain carbonates and bicarbonates that will raise the KH and pH.
Depending on how hard your water is and what level you want the GH, will determine how much of the Rift Lake conditioner you use. You can probably get away with using it at half strength to raise the GH of soft water to 200-250ppm. But you will need to check the directions on the pack and find out exactly what the GH of your current water supply is.
You will need to add the Rift Lake conditioner to any new water before it goes in the tank. The water and mineral salts need to be aerated and mixed for 24 hours before use so all the minerals have dissolved in the water. You can use a large plastic bucket or spare fish tank for this purpose.
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The KH isn't as important and is used mainly to stop the pH from dropping. The shells will, or should hold the pH above 7.0. If the pH drops then add more shells, limestone, or coral rubble ad monitor the pH over a couple of weeks. If it continues to drop, add more and monitor. Keep doing this until the desired pH is reached.
Shells, limestone and coral rubble can raise the pH to 8.5 if you have enough in the tank. You don't need a pH that high unless you keep marine fish or Lake Tanganyikan cichlids.
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Platies, guppies and swordtails do best in water with a pH above 7.0 and a GH around 200ppm.
Mollies do best in water with a pH above 7.0 and a GH above 250ppm. Mollies need the minerals much more than the other 3 types of fish mentioned and will suffer if kept in soft or acid water.