How do you clean the coral in a fish tank it's a 20 gallons deep?

If you are asking about live corals, they should not require cleaning unless water conditions are poor and nuisance algae are overgrowing everything. If this is the case, the various saltwater fish and invertebrates that do eat algae will not be able to keep up with this amount of growth. Correcting the water quality (eliminate phosphates, reduce nitrates, etc.) and verifying proper spectrum lighting will be the only longterm solution.

If, on the other hand, you are asking about dead coral skeletons, used as decoration the most common cleaning technique is to remove them, rinse or scrub them to remove as much algae as possible (no soap!!! water only) and then bleach them. Common chlorine bleach (e.g., Clorox) diluted (1 part bleach to 10 parts of water) will whiten the pieces back up. Rinse very thoroughly in fresh water and then allow to completely air dry before returning to the aquarium.