how does testing PH,DO and temerature in an ecosystem tell us the ecosystem's sustainability?

Ok, you're basically asking why certain abiotic factors are necessary for an ecosystem's stability. Abiotic factors can increase or decrease the amount of environmental stress on an ecosystem and therefore can also affect the stability of that ecosystem.

Let's stick with pH. Large changes in pH can drastically **decrease** species diversity and change species composition in freshwater systems. Why? Few species can tolerate such harsh environmental conditions. For instance, fish tend to be especially sensitive to changes in pH.

If you understand this, consider why acid rain has been a serious threat to freshwater ecosystems. Most fish thrive within a pH range of six to seven, but acid rain will decrease the pH. With a pH of 5, plankton and mosses may invade (these species prefer acidic conditions). Some species of fish have difficulty competing with the plankton and mosses for light and nutrients, and as a result their populations begin to drop. Below a pH of 5, entire populations of fish disappear, undecayed organic matter accumulates at the bottom of the lake or stream, and mosses dominate areas near the shore. Below a pH of 4.5, the ecosystem is essentially devoid of fish such as trout.

So again, the organization of ecosystems is based upon populations interacting with each other and with abiotic factors of the environment. If you change the abiotic factors, then you affect the stability of the ecosystem.

All of these are indicator tests of the system.  Plants and animals can only live within pH ranges and animals must have DO to breath.  Same with temperature.  They are indicator tests because if the numbers are bad, the system does not survive.  However, if the system numbers are good, it does NOT indicate the system is healthy.  There are hundreds of factors that impact a system and very bad things can happen, even if the pH, DO, and temp are within limits.  Thus these factors indicate bad, but if good, they do not guarantee good.