Two methods... you choose -
1. Go slightly brackish which is how I keep my mollies, and what I would recommend... hundreds of baby fry can't be wrong
Simplest method would be to add Marine Salt (like Instant Ocean) to bring the specific gravity up to 1.002~1.003... Marine salt is a mixture of many minerals and salts including table salt. It will raise both the GH and KH.
The guppy won't care one way or the other about the salt.
MOST plants tolerate marine salt up to 1.005 and your filter bacteria are fine at anything less than 1.005.....
Instant ocean marine salt is what I use and it is fairly consistent with 1 tablespoon per gallon = .003 raise... i.e. at the next water change mix roughly 3/4 cup instant ocean in a gallon container to dissolve it totally then as you are re-filling the tank pour it in gradually. Then you are at 1.002~1.003. Any future water changes, just mix 1 tablespoon per gallon for the amount of water removed and refill.
If you use a marine salt other than instant ocean the amount will be different.
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2. Just raise the GH and KH without adding common salt (NaCl Sodium Chloride)- not my recommended method, but workable.
You can raise GH by adding Epsom salt - start exactly 1 gallon of tap water and stir in Epsom salt (MGSO4 Magnesium Sulfate) for those chemistry people) a teaspoon at a time... dissolve it and measure the GH... keep track of how much it takes to get the GH where you want it. Roughly 120~180 for mollies. Write that down and you never have to do this again.
You can raise KH with baking soda (NaHCO3 Sodium Bicarbonate for those chemistry people) like Arm&Hammer)... same method as above, just mix it into the water from above so in the end you have a sample of what your tank water will be when finished.
Take the final measurements and figure out how much you need for your tank, then at your next water change, dissolve it in water and pour it in when refilling.
BIG NOTE: never add any sort of dry salt to a tank, if the fish eat if thinking it is food it can burn the gills chemically or worse... always dissolve dry chemicals in water.
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Either way, remember later if you do a water change and take out 15 gallons of water, you need to replace the salts you took out with the water. Also, if you are just topping off from evaporation you do NOT need to add anything except conditioner for the chlorine since salt won't evaporate.
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