nute burn/lock up IMO. I use RO but when my plants are that small they usually get ~150-200ppm in total nutes. I wouldnt be giving them 500 till they are in like week 3 or 4 of veg and they are ~12-18" tall. If you keep top feeding they might not look for roots and grow more leaves but no roots. They should have at least one tap root in the water by day 30.