Of late there has been a movement against the phase “fish diseases” in favour of “fish health”. And rightly so, since “fish diseases” is a negative phrase implying that it is almost inevitable  for fish to contract an ailment. Bearing this in mind, it has to be said that marine fish are far more likely to contact some sort of ailment than their fresh water counterparts, mainly due to the fact that the marine environment is so much more difficult to maintain.

PREVENTION

Preventing disease is obviously far more desirable than having a sick fish. The main causes of marine fish ill-health falls into three categories; Overfeeding, Overstocking and Stress. Stress is cause as a result of the other two factors, but there are many other unrelated causes of stress including bullying, bad handling, poor compatibility with other tank mates etc.
  The best ways to prevent disease are :

STOCKING-   Always stock slowly- the quicker a tank is stocked, the greater the risk of disease especially if in a newly matured tank.

FEEDING-   Feed no more than the fish can consume in a few minutes. Remove any excess without delay.

STRESS-     Marine fish are easily stressed, leading to a rapid breakdown of their natural immune systems and the subsequent contraction of disease
  Stress can also be brought about by high nitrate levels, unstable water conditions, loud external noises and toxic fumes within the room.

EARLY SIGNS-     Nearly all diseases will be signaled at an early stage by a change in the behaviour of the fish. A depressed attitude is normally a sign, as is hiding and or sudden shyness. This may be accompanied by sudden flicking or scratching against rocks, swimming erratically , rushing around the tank in a frenzy. If these signs are identified early then action can be taken. Action without delay is very important as many marine fish can become dangerous within 24 hours, making treatment more difficult.

DISEASES-     There are four types of diseases: parasitic, bacterial, fungal and viral. Often, one disease will complicate another.

   It I not normally discussed but many fish can experience physiological diseases just as humans or other animals. Heart, Kidney, liver and intestinal problems are bound to occur from time to time. The point to bear in mind is that if a  fish is ill, it may not be one of the common aquarium diseases; sometimes there is no effective treatment and a distressed fish may have to be destroyed humanely.
  The common marine aquarium diseases are; White spot, Black spot, Oodinium or Marine velvet, Gill and fin flukes, Head and lateral line erosion disease, Finrot, Marine fungus, Cauliflower disease etc.