Ppm's increase ?? Dont understand

my ppm is also going up daily from 300ppm to 400ppm then next da 500ppm. ive got an nft system, its a very small system like less than a foot long, i have 4 plants in their, and 3 r healthy but 1 isnt. iv tried everthing from washing out whole system with ph water incase of salt build up on spreader mat. ive thrown 40 litres of ph'd water over plants spreader mat and roots and system. ive even left it running with just water no nutes. i c that 3 of my plants r ok but 1 looks very deficient with drying up lower leaves. but the top leaves r healthy, it has roots sitting underneath the rockwool cube, so im not sure if the roots are root bound, even though there are roots from the same plant growing down the tray and at the sides of the rockwool slab.if 1 plant is root bound and not eating nutes the other 3 in the system should b, which is very strange. they r small just starting veg so they have not much of a root system an no roots r sitting at the bottom of there rockwool cubes. i hav soft water plant food but im not sure if i need hard water for plants to react with food.
 
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