How many gallons remain ?
Has it been topped up? With what ?
So many factors.
Measure - record - look for trends
I started daily tracking - tds, ph, temp ( leaf surface, atmos, res. )
Even with a lot of info, there are odd factors at play, that cause specific anomalies in every grow, whereby making generalisations intrepid.
I run a whitewater circuit, in 30 % rh, with low resivior volume. I can burn off a few gallons of water a day dry running (with no plants in the system) .
My ppm rises quickly as I lose water. I am running around 4-500 ppm. If I left it alone the ppm would be in the K's , by the time the pump started suk'n air.
You didn't ask me specificly, so a wild unsolicited scenario plays out like this,,,
As water leaves the system you are adding back solution @ the same strength. In this solution may be slightly stiff.
Can you measure ph ?
When in doubt - check the chart.
Hope this helps.
Hydro is fun, but it she bites at the slightest provocation.
Stay green. Have fun.