Monitoring water quality is the best way to know what is going on in your pond. There are tests that need to be run on a regular basis, at least weekly when things are going well and daily if you suspect problems. You need to test for pH, ammonia, nitrites, nitrates. You can buy test kits at pet stores that will give you these readings conveniently. The pH level for a pond is ideally between 6.8 and 7.4, but higher levels (even up to 9.0) are acceptable. It is more important that the pH level change as little as possible. Chemically altering pH levels is not suggested as it is pH swings that kill fish not the pH level. The hardness of water is the way it buffers the pH. In Utah we have very hard water so testing for hardness needs to be only occasionally or when fish are sick. Nitrate levels should be at 0 as should nitrites. Levels higher than this indicate problems with the ecosystem and danger to plant and fish life.
Once your pond has been constructed the work is not over. There are numerous tasks that need to be done on a regular basis.
What you should do