Antelope type dendrobium has yellowing leaves

[Joseph.joyner]

Hello all,
I have an antelope type dendrobium that I got recently that has been getting small black spots on its leaves. The leaves in question will then turn yellow and fall off. It is a beautiful plant with a full healthy flower spike but has been losing about one leaf a week.
The care I've been giving it is:
Near a south facing window out of direct sunlight.
A long drink about twice a week when the medium is dry
Fertilized twice this month.
I've attached a picture below. I would really appreciate any help you guys would be willing to offer, I love this orchid and would hate to lose it.
Thank you in advance!