Under gravel filters are sometimes a problem with keeping nitrAtes low 20 ppm. Gunk buids up under the plate and more frequent water changes along with vaccuming the gravel during water changes can help.
Most livebearers, Guppies,mollies,platys, prefer water with pH values of 7.4 to 8.0 they need hard, or basic ,alkaline water. If your water is indeed neutral, pH 7.0 ,,then I would recommend Marine salt at approx one tablespoon per 10 gal of water to be added slowly during water changes to the new water you add. Marine salt, unlike aquarium salt, will provide minerals that will increase the alkalinity and help to maintain buffering capacity.Regular aquarium salt won't. I would first get myself a test kit to see exactly what my water parameters are. I would NOT rely on fish store to provide this info. API Freshwater master kit is used by many hobbyist's and if your gonna keep fish,, it is in my view a must have along with a full function water conditioner such as PRIME, or AMQUEL+, Or MICROBE LIFT X-TREME,or AQUA SCIENCE ULTIMATE water conditioners. Hope some of this helps.