Most aquarium stores can check your water for you. We don't advocate the use of chemicals beyond those needed to correct the water quality from the tap. Natural additives like bottled bacteria and salt are fine to add. Pond salt is good for fish in small doses, especially Koi, but can be bad for plants. Make the pond as large and as deep as you can afford to.
Many people find thier pond to be too small.
The pond is also much healthier and more stable with a larger volume of water. Ponds are actually require less maintenance when they have more volume. Don't put a pond on low ground, in a wet spot or other area where runoff from rain will interfere with it. Don't put a pond under a tree with poisonous nuts or berries. Don't put a pond on ground that is unstable or shifting.