Quote Originally Posted by cvonseggern
That may well be the issue - I know my RO/DI unit is overdue for a cartridge change.
Toping off with RO/DI, even one that isn't working great, should be keeping your TDS down. Folks that top off with declorinated tap water can get a HUGE TDS buildup over time. It is probably just high Ca (450-500) and your test kit isn't acurate at that level. Try testing with 1/2 Salt 1/2 RO/DI and see how your test kit does with that?

When you can, pick up a Salifert Test Kit - I saw they had some under the counter at Alamo Aquatics yesterday!