Lol. 600 ppm's is equal to 1.2 EC on the 500 scale.
If your 1500 is actually an EC reading as opposed to ppm's.......your EC is 1.5. Which is equal to 750 ppm's on the 500 scale.
When I see 1500........it can't be an EC reading.
Never flush with plain water.
If you must flush due to salt build up........use a light nutrient solution.
Feed at least once a day to substantial run off.......with a pH around 6.0.
Never use plain water.
Never let the coco dry.
Good luck