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Marine Fish Head and Lateral Line Erosion

Cause, Cure, and Prevention

Pacific blue tang with HLLE


Published - 20051203, Revised:

Head and Lateral Line Erosion (HLLE) is a common condition seen in marine fish kept in public and home aquariums. Symptoms are pitting on the head and deterioration of skin and scales along the lateral line. The most common fish to show the condition are tangs (surgeonfish), butterfly fish, and marine angelfish, but I have also seen the condition in other fish, including clownfish. This condition only happens in aquariums that have been set up for months or years. Through experience and changing environmental conditions, I have halted and reversed the condition.

Since having built and operated Aquaworld Aquarium for the last five years, I have seen many cases of HLLE in many marine fish species. At Aquaworld, I sometimes have fish with HLLE brought to me by hobbyists that had their aquarium set up for years. I would take these fish knowing they had little chance of being sold, but I would try to provide them with the best care possible.

The first step was quarantining fish for three weeks and then introducing them to one of my reef systems. Over time (usually several months), the HLLE would completely reverse, and you could never tell that the fish ever had the condition. Through talking to customers with fish with the HLLE condition, I have been able to correlate it to a common aquarium condition. The common factor in all cases of HLLE in marine fish I have seen is high nitrate, often over 400 ppm.

While it could be some other environmental factor/s that causes the condition, I have never seen the condition in marine fish that were kept in well-maintained reef aquariums with 50 ppm or less nitrate. In all cases I have seen, the nitrate was always above 100 ppm. Exactly what concentration of nitrate the aquarium needs to be at before HLLE begins to appear is different depending on the species. Purple tang (Zebrasoma xanthurum) appear to be the best barometer for letting you know that your nitrate is too high, followed by Pacific blue tangs (Paracanthurus hepatus) and sailfin tangs (Zebrasoma veliferum).

In the past, I have seen the condition pop up within just a week or two in some species of tangs (sailfin [Zebrasoma veliferum], Pacific blue [Paracanthurus hepatus], and yellow [Zebrasoma flavescens]) and butterfly fish (Heniochus acuminatus) in a fish only system where nitrates had been above 400 ppm. These fish were provided excellent diets, and the condition still developed. After performing a 75% water change on the system, dropping the nitrate to less than 115 ppm, HLLE no longer develops in these species. Massive water changes from 50 to near 100% will not only stop HLLE from worsening but will reverse the condition if the nitrate is maintained below 40 ppm.

Purple tang (Zebrasoma xanthurum) with HLLE
This purple tang (Zebrasoma xanthurum) was completely cured and then sold.

While nitrate is always present in high concentrations when HLLE develops, it may not be the actual cause, but rather a sign that the water quality has deteriorated beyond an acceptable level, and some other water parameter/s that accompanies high nitrate water is out of acceptable level/s. Without a controlled lab experiment where nitrate is purposely introduced to the tank and all other factors have been monitored and eliminated, can I say nitrate is the definite cause, it is where I would look first before looking elsewhere.

In all cases of curing HLLE, no special change was made to the diet of the fish. Providing the fish with water with low nitrates and/or doing large percentage water changes to keep the nitrate low (below 40 ppm) always reversed the condition.

While it is possible to reverse HLLE, it is a slow process and can take months. The best practice is to prevent it from happening with proper maintenance. Start with an appropriate aquarium setup to prevent HLLE from developing. A Deep Sand Bed (DSB) consisting of fine aragonite (coral-based sand) of 2 to 3 inches will help control nitrate accumulation by providing an environment for anaerobic bacteria to establish and break down nitrate. A DSB can also help maintain calcium and alkalinity levels. Using a refugium with macroalgae as a filter will also help reduce nitrate, and in some systems, it can help keep it near 0 ppm or undetectable.

Deep sand bed for controlling nitrate in marine systems
A Deep Sand Bed (DSB) helps control how fast nitrate can accumulate in the aquarium. Whether you have a fish-only or reef aquarium, a DSB can be very beneficial.

If your current aquarium eco-system produces a lot of nitrate every month, a large percentage of water change to reduce the nitrate must be performed. How much will depend on how fast nitrate accumulates. Small percentage water changes of 25% a month will only reduce the nitrate concentration by 25% (provided the new water has no nitrate). If you aim to maintain a nitrate level below 50 ppm, and your system accumulates 25 ppm every four weeks, you must change 50% at a time.

Biweekly water changes of 25% will not reduce the nitrate by 50% in the same four-week period. If you continue this practice, your nitrate will climb above 50 ppm. After just two water changes, your nitrate concentration would be around 47 ppm after completing the second water change.

Large percentage water changes will not adversely affect the animals of the tank as long as the new water is well mixed and the salinity and temperature are close to the aquarium values. Generally, I never do less than a 50% water change, even in aquariums where the nitrate reads undetectable.

When taking water out of the aquarium, never gravel vacuum the sand bed. This will upset the ecological balance of the bed, removing beneficial organisms and destroying the anaerobic layer. If detritus accumulates on top of the sand bed, just pull it off the top. Adding a small power head (wave make) that creates a little more current in the tank will help keep the bottom free of detritus accumulation.

While HLLE appears to be related to high nitrate, and not nutrition, internal parasites, or stray voltage. Providing proper environment and nutrition for the type of fish you are keeping should not be ignored. Ensuring the fish have a good diet may shorten the recovery time.

I hope this article clears up some written misconceptions about the cause of HLLE in marine fish and provides you with a real solution to the condition. Don't forget to do those large water changes!


Since the original publication (2005) of this article, research published in 2020 shows a direct relationship between nitrate and methemoglobin: Daniel F Gomez Isaza, Rebecca L Cramp, Craig E Franklin, Simultaneous exposure to nitrate and low pH reduces the blood oxygen-carrying capacity and functional performance of a freshwater fish, Conservation Physiology, Volume 8, Issue 1, 2020, coz092,