Swamps,
Which reading is it, the 'in' one or the out one?
I think, if I remember other postings that you have Peter's System, the same as myself.
If you have fiddled with the probes (as I did) you must make sure that the 2 tiny dots, 1 on the probe and 1 on the mounting are perfectly aligned. Peter told me that you can also get something he called 'TDS creep' giving a false reading.
Mine settled down fine.
A further check would be to also have a hand held meter.
Did you have the hard or soft water setup? Mine was the hard water setup, therefore I had the RO membranes and the DI resin too.
If yours is the same, what is your 'in' reading and your 'out' reading too?
You can always ring Peter or mail him and he can explain things fully then.
Ian