There's no way you went from 160 ppm to 0 ppm nitrates in 4 days. That would be almost impossible to accomplish. What test kit are you using? Now you feel like you've kept your tank too clean, yet you say you had 160ppm nitrates? Is this post for real? If it is, I would suggest getting some good test kits, or taking your water to a local fish store to be tested and start there. The acans could be doing poorly from any number of things, including but not limited to, bad water conditions, light, flow, acclimation, etc... What other corals do you have and are they doing well? Start with getting some dependable test numbers and go from there! Hope you get it sorted out.