I would be less concerned about how much ppm is being reduced daily as I would be concerned about total volume in the reservoir being consumed, somewhat constant/steady ppm numbers indicate your feeding where the plants want to be or at least within a range that doesn’t lock out any vital nutrients, if your volume is the same and ppm is rapidly raising or falling, you have PH out of wack or a nutrient lockout of some sort that needs to be addressed, when numbers start changing drastically one way or another, I would flush whatever you can out with clean water and let the clean water run for a half hour at least, dump, refill with clean water and flush a second time before dumping and refilling with corrected nutrients and PH. If you suspect bad bacteria causing issues or getting slimy roots, when flushing add 2 drops of bleach to every gallon, and when refilling with nutrients add the same amount. This will ensure any bad(all will actually) bacteria dies off for a fresh start.. Not a botanist but have setup quite a few different hydro setups and learned a decent amount from each one.