What are two similarities in unhealthy and healthy coral reef

By unhealthy, do you mean dead?

IF you mean dead, a unhealthy reef doesn't contain that much fish in that area. The corals are dead and have a grayish- brownish color. This places are usually hunting grounds for sharks. A healthy reef contain fish that are necessary to keep the coral alive and healthy. Fish such as the Parrot fish are in charge to eat algae and dead coral. The similarities I see here is that fish still roam around in those areas, and sharks do also, but not as much.  

Explanation:

In a healthy coral colony no parts are affected by disease or bleaching. ... the living/healthy coral tissue and the diseased portion, in comparison to coral

Explanation:

succession returns a disturbed

ecosystem to a more stable state. this is also classified into two (2) types:

primary succession and secondary succession. the primary succession arises when

a new land is transformed and a bare rock has been exposed providing an environment

that can be occupied for the first time. while, secondary succession occurs

when a previously occupied environment was disturbed that kills much of all its

community

explanation: