Reef tanks evolve over time. Some can maintain a subclinical level of disease maintining a low amount of redbugs, macroalgae, or even cryptocaryon (marine ich) but eventually these things may overwhelm your system (usually while on vacation). This section explains options for erradicating these nuissances.

How Severe is the Problem?

If multiple fish are dead/dying or the water is visibly cloudy, consult the emergency section.  

Verify Water Chemistry Levels

Assuming it is not an emergency, we want to narrow down the causes.  First, let's confirm that the water chemistry is fine.  This is one of the most common underlying causes and one of the easiest to fix,  Make sure you have run all the regular water tests.  

Review Common Symptoms

If water chemistry is fine, you may be facing an infection or parasite.  Second, let's view the most common symptoms and see if you recognize any of these in your fish.  A short list includes:

  • Disease of the mouth
  • External infections on the body or fins
  • Respiratory (gill) Problems
  • Specific problems with the eyes
  • Visible Spots on the body

Who is effected in general?

In addition to assessing the symptoms, we also need to consider how long the affected fish has been in the tank.  If nothing new has been added in more than a month (and especially if only a single fish is affected), the most common problems are:

  • nutritional deficiency
  • stress or injury from being picked on by another fish.  See Fish Compatibility
  • overnight fin or body lacerations or circular lesions (bites from blennies/triggerfish/pufferfish.  See Fish Compatibility
  • head and lateral line disease
  • one sided eye cloudiness or swelling +/- gas behind or above the eye

If a new fish has been added within the past month and that same fish is affected:

  • The fish could have been starved prior to purchase.  Small fish (<2 inches) are more likely to starve because they have less fat stores.  Notorious examples are <1.5” angelfish and <2” butterfly fish
  • It is possible that the fish was caught with cyanide
  • The fish may have intestinal parasites, which is especially common in high metabolism fish such as butterfly fish and tangs.  Most wild fish actually have gastrointestinal parasites. 
  • The fish may be getting picked on by dominant fish.  See Fish Compatibility

​Do not take action before you have identified the problem.  Unless you are certain you already know what it is, please read more about disease identification.  


Before exploring treatments, it is important to realise that treating sick fish without identifying the cause of disease is not ideal. Sometimes even doing a water change may make the fish more sick. In order to properly diagnose a disease, fish often need to be sacrificed (for the good of the herd). In aquaculture and veterinary medicine, diseases are usually identified using this approach. It's understandable that sacrificing a few for the good of the herd doesn't work when you only have 1 sick goby or lionfish, and, unlike the case with a dog or a tiger, fish are generally more fragile and difficult to work with. Even if we could get the right samples, the disease identification and therapeutic tecniques are not as well developed as in other areas of animal medicine. There are also times when we can't even catch the fish in the first place (see section on catching fish). Other times when the fish is so fragile/sick/small, that a diagnostic tecnique would probably hurt the individual trying to be helped. These things must be considered and weighed depending on the case. Use your best judgement and the resources available on this site and others (hopefully credible) to make informed decisions about diagnosing and treating your aquatic animals

Please remember that it's possible to do more harm than good. Entire systems can be thrown off cycle when antibiotics are used without realising the implications on the "good" nitrifying bacteria. Other treatments, like copper/formalin/chloroquin, can kill just about anything if the concentration is high enough.

That said, parasites can be treated through a variety of drugs and also non-drug medications, some of which work well on a variety of parasites (copper and formalin for example)

Treating bacterial infections generally require a licensed aquarium veterinarian.  Here is a list of common medications and where to find them

Reef Parasites and Nuisance Animals

There are a variety of pests that can affect reef aquariums.  Some aquariums can maintain a subclinical level of disease maintaining a low amount or redbugs or a macroalgae or even cryptocaryon(marine ich) but eventually these things may overwhelm your system (usually while on vacation). Click here for the full list or check out individual parasites/nuisances below.  

Marine Animals Diseases 

Below is a list of some of the most common diseases and symptoms