help #chalice

Hello everybody, FNG here to R2R forums. I have a 15 gallon AIO that I have used to experiment and try out reefing for the first time bought it 2 years ago. (The tank was a 100 bucks, why the hell not give it spin.) I have a lot of freshwater experience, and once I mastered that I thought...

So, I am loosing tissue on a large chalice coral in my IM15 G. Nano tank doing 10% weekly wc. I have a crappy light and the LFS kept the chalice in roughly 100-150 par. Mines at maybe 50. I am doing my water changes but the chalice seems to get worse and worse every week, any tips on care...