from experience with fish, not orchids I would have to ask if algae is
a problem in your tank and if you are using real plants. If you
medicated your fish or use algae killer that could be a problem for any
plant. My fish tank has a high nitrogen content in the water, so little
things like that could affect the plants, also. I'd get the water
tested at a petco or petsmart to see if everything is balanced, and if
your aquatic plants are ok, it should be alright for orchids.