the times i have seen this happen to acans, it was always one of the following:

1. temp - this doesnt seem like your issue considering your posted parameters
2. too much light - acans dont like sunlight very much - might be your issue, dont judge based on success of chalice or favia as they can sometimes handle more light
3. something picking on them

i am not saying it couldnt be something else, that is just my experience.