I obtained a mature Brassia orchid as a gift about two months ago. When I first recieved it everything looked fine. It had a healthy spike of flowers and about a month later sent out three others that are now beginning to bloom. It is currently in my bedroom and recieve's shaded light (due to a tree) from a south-west window. The temperature in my room reaches the high seventies. I keep it's pot on a tray of pebbles filled with water to increse the humidity. I've watered it once every week and fertilized it once every four weeks with a 18-18-18 solution. Everything looked fine the first month and a half since recieved it, but about a week ago I noticed three edges of some healthy leaves turning a brownish-yellow. I was certain this wasn't caused by the light because it's not that bright to burn it. I noticed that it was getting a blast from the air-conditioning and I recently moved it from that place. It's now a week later from that time and I've noticed four more of the tips of the leaves are beginning to turn this browish-yellow. What could I be doing wrong? How do I help this or prevent this from occuring again?