first thing is do you know how much water you've displaced with substrate and live rock. if you're rock is very dense and you have lots of it then you may only have 30 gallons or less of actual water in the tank. if thats the case then 15 gallons ( or 50% of a total of 30 gallons) would be a little high. even if you have 45 gallons of water in the system this is still over 30% water change, which is quite a bit. its troubling that you're still reading nitrates despite these large and frequent water changes. i would recommend checking your water source for nitrates. its possible that your local tap water has some nitrates, or if you're using ro/di then perhaps your filters need replaced??? if your water source has no nitrates then you need to implement/improve some method of reducing your nitrates. a refugium with lots of macro algae is a popular method of eliminating nitrates, but others exist as well. while 40ppm of nitrates will likely not prove immediately deadly to most corals (although most anemones are much more sensitive to this than coral), i assure you if your fix your nitrate problem your corals will open/color up much nicer and their rate of growth will improve.