DI water is water, with all chemicals and contaminants removed, it can usually bought at the grocery store.
When I worked as a chemist I rinsed all sample tubes 3 times in between tests with DI, to remove any residual. Without those rinses, any tests performed were considered invalid.
Because you are dealing in Parts per Million, and using only tiny amounts of sample, any residual carry over can effect numbers big time.
If your water provider uses chloramine as a disinfectant, ammonia with show up in low levels as "noise" in a test. If you rinse with that same tap water, it may skew your results.
At 3 years have your regents expired, if so toss them and get fresh.
In the lab we always run a DI sample, a known standard (2ppm) and the real sample (in the photo below my tank water) DI is yellow, tank is little darker yellow, the green is a known 2ppm standard
