Is it a handheld TDS or an inline?
If an inline - what reading are you getting on the 'in' side? Assuming the 0.24 is on the 'out' side.
If you changed ALL of the filters and the membrane (especially) - you need to let it run for hours for new membrane to begin working (and to get all of the carbon out of the new carbon filter)
Any other reefers nearby that can check against their TDS for you..or even a LFS?