Ammonia and nitrite should always be zero. Nitrate is variable because there could be some in the tap water. Generally speaking, nitrate should be no more than 20ppm above the level in your tapwater, and it should be kept below that level by water changes.
pH, general hardness and carbonate hardness do not need to 'kept' at any particular level. The readings for your tank are determined by your tapwater. Trying to alter these three is not something to attempt lightly as there is the potential for things to go drastically wrong and kill your fish. It is better to choose fish that suit your pH and hardness rather than try to alter the levels. Both pH and general hardness (GH) are the ones to look at when choosing fish. Carbonate hardness (KH) does not affect the fish directly but is a measure of the buffering capacity of the water. If KH is low, there is the potential for the pH to drop, which could be bad for the fish. But a medium to high KH indicates the pH will be stable.
You say you have platies - they like a pH over 7 and hard water.