I guess most had been explained as above.
If you want to increase your water hardness(GH), you can try to get some GH+ salt which are actually minerals.
Don't use salt which is Sodium Chloride.
Here are two suggestions for you:
1)You can try to add coral chips which is the cheapest but they are hard to control/monitor in your tank.
If your water GH fluctuates too much, it will be bad for your fish.
2)You can use Seachem Cichlid Lake Salt. It actually contains Calcium, Magnesium, and other minerals.
Check your water GH as
@Naughts had mentioned.
If the GH is much lower than the require GH for Guppies, you can use this Cichlid Lake salt to increase the GH.
It's better to premix the Cichlid Lake Salt and your tap water in a bucket before putting them into your tank to prevent fluctuation of GH in the tank.
I attached some information here about Seachem Cichlid Lake Salt.
www.seachem.com
Guppies info:
www.seriouslyfish.com