final week

Did you use organic or synthetic nutrients? If you used organic there is really no need to 'flush' as you would effectively have to flush away all organic matter and microbial life which is practically impossible - just stop adding nutrients and water normally (always pH) until the end. If synthetic nutrients were used then you will need to basically try to cleanse your substrate and flush - water to say 10-20% runoff, wait 5 mins to allow the nutrients to get picked up by the water, water to runoff again - do this over the next few days until the ppm of your runoff is like...50ppm and then you are pretty much there.