m0useanswered grow question a year ago If your not topping up the reservoir with more water to keep it at the same level then it will get concentrated. Thus ppm will go up
Eg, in a very basic unrealistic easy to understand scenario, you start with 10% feed in 10L, over the course of 5 days the plants eat 2% of the feed and water evaporates/gets used up by plants at a rate of 50%
Now the feed left in the solution is at 18%
10%-2%=8%
But the water is half the volume from original 10L, 5L goes into 10L twice
8%*2=16%
In short, add in water as its getting used.
Or change the amount of feed your adding in for the needs of the plants.
Good Luck!