*This is assuming the water level is remaining more or less the same*
A falling ph with a rising EC is not the time to start adding just water.
I will explain why.
Your plants roots sort of have their own internal EC, which will be different from that of the water/nute solution.
So if the plants internal EC is LOWER than the solutions, the plant will absorb nutrients along with water.
If the plants roots however have a higher EC than the nute solution, they will leech nutrients and raise your nute EC.
So, if you have a static water level, with a rising EC, raise your nute EC.
If your water level is dropping and the EC is rising, this is when you need to add water to the nute solution.
This has not looked at the ph.
If your ph is falling in DWC, it generally means your EC is too low (as above).
There is a complication however and that is your air quality.
When pumping air into a DWC bucket, you are pumping in Oxygen but also CO2. The addition of CO2 to water in this way is exactly the same process responsible for acid rain. (Power stations pumping CO2 into the air, which goes into clouds and hey presto, acid rain".
So, if you are in a city, with high levels of CO2, this may be the cause of falling ph.
A res change, maybe even twice usually sorts this out.
So, the solution to this depends on your water levels.
W