Carbon dust is a prime cause of HLLE. Reversing it may not be possible once the lesions are deep enough. Moving the fish to a tank that has never held carbon works. Here is an excerpt from my upcoming fish disease book that discusses my study:
The activated carbon connection
A relationship between the use of activated carbon in aquariums and the development of HLLE in surgeonfish has been positively shown in two scientific studies. Other than that, no formal studies have been undertaken that identify other causes. However, a multitude of unproven causes have been presented by various people. Commonly, stray electrical currents and vitamin deficiencies are cited as causes, but one of the studies mentioned above ruled these out as common causes.
Based on those preliminary observations, the Toledo Zoo performed a study that demonstrated that, indeed, lignite carbon causes HLLE in surgeonfish. Minor HLLE lesions were noted on two study fish 20 days after the lignite carbon was added to the sump of their system. The lesions began to develop on the additional fish in that system and grew in size until after four months, all of the fish in that system showed severe lesions (eventually involving over one-third of their body surface). The control fish did not develop any lesions.
The carbon study hypothesis
The basic hypothesis of this study was that activated carbon causes HLLE in fishes. Originally, it was thought that carbon dust (known as fines) was the causative agent. This was based on observations at the Toledo Zoo where carbon was removed from an aquarium and the water was changed, yet HLLE symptoms could still be produced by adding susceptible fish to the aquarium, indicating there was some unknown residual action by the carbon.
Carbon fines were frequently discovered in the filter sumps and substrate of these tanks. Changing all of the aquarium’s water, decorations, and substrate would then render the aquarium safe for housing susceptible fishes (assuming no new carbon use).
Two public aquariums have reported acute outbreaks of HLLE in systems where carbon had been accidentally ground up and ejected into aquariums by mechanical filtration systems. It has also been reported that aquarium systems that use protein skimmers (foam fractionators) do not seem to develop HLLE as frequently, even when carbon is routinely used. Since protein skimmers remove particulate organic carbon from water (including carbon fines), it was thought that this might be the reason these systems do not cause HLLE as readily.
In addition, the hard-pelleted carbon used in this study did not cause severe HLLE, while the soft, dusty carbon did. However, no carbon fines were seen during histological examinations of the lesions of the study fish. This means either that the dust causing the effect is fleeting, the fines were too small for the histologist to see, or there is some other factor associated with carbon use that causes HLLE in susceptible fishes.
This effect has even been seen in freshwater fishes. In one case that demonstrates this syndrome very clearly, a pair of juvenile Australian lungfish were quarantined for over six weeks and then moved into an exhibit tank. They showed no signs of any problems. After a month or so, some driftwood decorations had caused yellowing of the water, so activated carbon was added to the power filter. The particles were small enough that they escaped the carbon container and were injected into the water. Within a few weeks, the lungfish developed unmistakable HLLE lesions. As the condition worsened, it was decided to remove the carbon from the filter and change all of the substrate (as it was contaminated with carbon dust). Within four weeks of removing the carbon, the HLLE lesions were seen to begin shrinking and within two months, the lesions were completely healed. Since these fish were actively growing, young fish and the lesions were noticed almost immediately, the problem was correctable. In aquariums where the fish are mature, the problem was not identified early enough, or where the carbon fines could not be 100% removed, it is unlikely that the aquarist will be able to resolve this problem.
Carbon recommendations
The recommendation based on the clear effect that the use of carbon had on the study fish is not to use activated lignite carbon in aquariums housing fish species susceptible to HLLE. Other means of water quality management should first be explored, including water changes, non-carbon chemical filtration, or protein skimming. Extruded pelleted carbon may be more suitable, especially if used sparingly. No conclusions can be drawn regarding the use of carbon filtration products that were not tested.
If you do use carbon, rinse it well in reverse osmosis water prior to use, use a protein skimmer, and do not place the carbon in a high-water-flow reactor (that might serve to break the carbon granules up into finer particles).
One special case seems to be the HLLE-like problem seen in cichlids (notably discus, angelfish and oscars). In these cases, the malady seems related to a flagellated protozoan in the gut, possibly Hexamita sp. If caught soon enough, 10 mg of metronidazole (Flagyl) per gram of food (such as in a gelatin food mix) fed for five days may clear the problem. If the fish are not feeding well, metronidazole can be added to the aquarium’s water at a dose of 5 ppm every other day for three treatments. If the syndrome is not caught early enough, the HLLE-like lesions may become permanent. In addition, metronidazole is very bitter, and some fish will not accept it when added to their food.
Jay
