Hi Katherine,
Fluid bed biological filters such as yours can be cleaned of any trapped detritus by placing the sand/media in a fine mesh plastic strainer, and gently pouring some of the tank's warm saltwater over this media, swishing it around somewhat to dislodge any unwanted debris. Rinse well with this tank water, but work quickly and return the media back into the filter and place back in service after rinsing.
When doing a scheduled water change on your tank - use some of this discarded tankwater for the rinse.
The key is to protect your bacteria, that form a slime on each particle of sand/media, without shocking them or killing them off with chlorinated tap water ( freshwater ).
These types of filters, fluidized beds, do not require much cleaning, due to the fact that the media is in constant motion when in operation, and much of the debris should be dislodged under this normal operation.
A quick visual should let you know if you even need to clean it.
No debris or noticable crud - I would not clean it.
If you feel you must - once again - protect your bacteria, warm tank saltwater rinse only if absolutely necessary, and work gently and quickly.