I don't think guppies like brackish (salty water) They are a very hardy species, but you do need a heater. I have kept Guppies succesfully for several years...but never much paid attention to water conditions, as they are so hardy.
I took this from the web, and sounds like good advice to me.
Water pH

The acidity or alkalinity of water is measured in terms of pH of 0. If it is 100% acidic, it has a pH of 14. A pH of 7 has no measurable amount of alkaline or acid, that is, it is neutral. The best pH for guppies is 6.6 to 6.8. Most guppy keepers, however, do not pay much attention to their pH levels.
Water Hardness

The degree of water hardness (DH) depends on the amount of dissolved salts. Rainwater and distilled water have very low levels of dissolved salts. Hard water (also referred to as lime water) could be dangerous to certain species. If you use gravel in your tank, remember that it will gradually dissolve in the water over time, and build up the DH level. If a white scum line starts to build up at the water line, this is usually due to distillates, which are a product of dissolved salts. You should try to determine the cause of the sudden rise in the DH level and change the water immediately.

They also thrive in water in the mid 70's fahrenheit.