Fish Disease Symptoms Aren’t Always What They Seem

It's important to strike a balance between quick intervention and overreaction when it comes to marine fish disease symptoms

It’s important to strike a balance between quick intervention and overreaction when it comes to marine fish disease symptoms

Most marine aquarium hobbyists keep a close eye on their fish for certain tell-tale signs of ill health. And that’s a good thing, since quick intervention in the case of fish disease can often be the difference between life and death for the specimen(s). On the other hand, we do need to be cautious about overreacting to every suspicious visual or behavioral symptom because sometimes these warning signs may not be what they seem.

Remember, if misapplied, medications and therapeutic protocols for fish can do considerably more harm than good. It’s important to have a fairly high degree of confidence in your diagnosis before proceeding with treatment. That means you have to guard against misinterpreting normal behaviors or forgetting that more than one problem can cause similar symptoms.

To help illustrate this point, here’s a sampling of symptoms that may or may not spell trouble for your fish depending on the context:


If you’ve ever been through a major outbreak of Cryptocaryon irritans, no doubt the sight of a fish turning on its side and scraping its body on the rockwork causes your heart to skip a beat. And, indeed, flashing is a potentially worrisome symptom. However, this behavior doesn’t automatically signal the presence of a parasite or other problem. From time to time, many perfectly healthy fish will exhibit this behavior—what I can only assume to be the fishy equivalent of scratching an itch (though not necessarily scratching ich).

From my experience, this behavior is somewhat common among various wrasse species. The first time I noted it was in a bird wrasse (Gomphosus varius), and it nearly made my head explode because the specimen had been properly quarantined for a month and never showed any sign of disease. Fortunately, the behavior never intensified and the fish appeared pretty healthy and calm otherwise. I decided to simply observe the specimen, which turned out to be the proper course because no disease ever materialized.

Pitting on the head

Pitting or tissue loss on the face and head of a fish can be a sign that head and lateral line erosion (HLLE) is developing. But such isn’t always the case. Sometimes this symptom is merely a mechanical injury that will heal in short order if the specimen is provided proper water conditions. Other times, it can be a natural characteristic of the species.

For instance, the Pacific blue tang, aka the regal tang or palette surgeonfish (Paracanthurus hepatus), naturally develops freckle-like markings on its face as it ages, and these can easily be mistaken for the start of HLLE. Of course, there is a complicating factor to consider here: Captive specimens of P. hepatus also happen to be highly susceptible to HLLE, so you have to observe them carefully and try your best to discern between those natural markings and the start of something more sinister.

White spots or “growths”

Sometimes white spots or lumpy growths that suggest a disease such as ich or lymphocystis are actually caused by excessive mucus production—with the mucus either trapping bits of debris or forming clumps that cling to the fish’s body or fins.

In this circumstance, there’s still likely a problem to deal with because, unless the fish just happens to be a naturally slimy species, something is causing the excessive mucus production to begin with. Of course, various diseases can cause this symptom, but so can a wide range of water-quality issues. Obviously, treating for disease when the problem is out-of-whack water parameters won’t do your fish any good.

“Net damage”

Many years back, I purchased a small group of green chromis (Chromis viridis) only to have them die one after another in quarantine. The dead specimens exhibited a linear bruise across the body posterior to the dorsal fin that looked suspiciously like net damage. Aha, I thought to myself, these must have succumbed to rough handling inflicted at some point in the supply chain. But with the benefit of having read and edited Jay Hemdal’s The Salt Smart Guide to Preventing, Diagnosing, and Treating Disease of Marine Fishes, I can now be fairly confident those “bruises” on the chromis had nothing to do with being netted. Instead, they were likely one of the characteristic symptoms of red-band syndrome, caused by the protozoan Uronema marinum.

In the section of the book dedicated to red-band syndrome, Hemdal writes, “Almost universally, aquarists who do not have access to a microscope will identify this disease as a ‘secondary bacterial infection resulting from some injury.’ In fact, ‘capture damage’ is often cited as the original cause due to the often linear nature of the lesion, which looks much like a bruise from being hit with a net frame, for example. The rapid onset of the lesion (often many days after capture) and the fact that it develops internally and then erupts externally both point to another cause.”

A fish lying on its side

It would seem to be a safe bet that a fish lying flat on its side is in pretty bad shape—either on its last legs (fins?) or already “checked out.” But sometimes a fish in this orientation is still alive and well but “playing possum” for some reason.

I’ve seen this in various triggerfishes, usually when they’re newly introduced to a system or when their tank contains minimal rockwork. Perhaps this behavior is a stress response or an effort to keep a low profile when they can’t wedge themselves into rockwork crevices as they would in nature. But whatever the motivation, it can be hair-raising waiting to see if the specimen will eventually come around.

What’s the takeaway?
My point in all this is not to downplay worrisome symptoms or discourage hobbyists from responding in a timely manner to fish disease. Rather, I’m merely emphasizing that taking the right course of action—assuming any action is even necessary—is just as important as initiating prompt treatment. And that depends on making the best possible diagnosis based on all the available facts.


If you enjoyed this post, subscribe to get our new posts in your email.
About Jeff Kurtz

Jeff Kurtz is the Co-founder/Editor of Saltwater Smarts, former Senior Consulting Editor for Tropical Fish Hobbyist Magazine, and the aquarist formerly known as “The Salt Creep.” He has been an aquarium hobbyist for over 30 years and is an avid scuba diver.


  1. Irakli Shavgulidze says

    Dear Jeff,

    Thanks for this article, which was interesting and useful as always.

    I’m quite new in the hobby but still want to share my observation to confirm what you mention about non-ich-related flashing being common in wrasses. When I first noticed this behavour in my yellow wrasse (shortly after being introduced into my DT having been through a full quarantine period involving copper treatment), I nearly got a heart attack. With closer observation I also noticed that he was seemingly feeding on something from the rock – he would
    splash against the rock and than catch something I could not really see. Later I saw a large wrasse doing a similar thing in the wild in a National Geographic documentary.

    I have also observed my cleaner wrasse scratching against a rock very intensely and than a piece of his blue scale (he’s the only blue coloured fish in my tank) floating around. A day before I saw he was stung by one of my tube anemones. So I hypothesized the wrasse simply was trying to shed the damaged scales.

    These are just what I thought my wrasses were doing. There may be other explanations.

    Best regards,


  2. I had the same thing happen with my Eibli Angel, it looked like he was flashing all over the place and, I immediately thought it was ich but I never noticed any spots or any other tell tale signs that commonly go with it. Not wanting to risk the rest of my tank getting it, I was preparing to move him to my QT tank and as I watched him a lot closer to see where he was actually “itching” to see if I could spot anything at all. I realized that he actually wasn’t flashing, he was scraping coralline algae off of the live rock. I had noticed some of the coralline had disappeared but I thought it was just natural die off…turns out, the angel was the responsible party. I just keep a little more died red seaweed in the veggie clip now and it has subsided (both the mysterious coralline “die off” and his “flashing”).

Speak Your Mind