Hi Dominic, If I was in your shoes I would remove the discus to a bucket, partial empty the tank. Remove the gravel and plants (you may be amazed at the amount of junk that collects in the gravel). Empty and clean the tank. Add an 1/2 inch of sand (be sure to pre-rinse) and a refill the tank with clean water. Move the discus back into the tank. Be sure to temperature check the tank water to the bucket water.
Pat