HELP! Coral keeps dying!!

Hello i currently have a hexagon tank that is 35 gallons with 2 clownfish 3 chromis and a red skipper in it. I also have alot of coral in it. Recently i have been having a problem with some corals. I have lost a frogspawn, 2 brain corals, and acans. I tried moving those all around the tank when i saw them suffer but this did not work. I have a kessel 350 on my tank. I had everything tested in my water and it all came out perfect. I also have some stony corals in my tank and those are doing fine along with my zooanthids. I do water changes once a month which probably could be alittle more often but im sure that wont kill my corals right? Any help with this would be greatly appreciated. Thank you so much