Oscars have been captive bred for 50 years or more. I had a tank bred albino 30 years ago. The last oscar I had lived 14 years in my tanks, it just died last year. It was a full grown rescue when I got it and it had severe HITH. It took about a year to get the HITH cured up, which I did. Carbon, heavy metal poisoning, poor water quality (especially high nitrates), and poor diet (imbalanced Ca/P/Mg levels and vitamin C deficiency) are the usual accepted causes, or "suspected" causes more appropriately. It's still not clear whether bacteria and flagellates associated with the two are the cause or opportunistic colonists of the open wounds, although infections of both Spironucleus vortens and Hexamita salmonis have been proposed as causative organisms. To that effect, studies of seasonal infection rates of HITH in wild populations of east African species of both Oreochromis and Tilapia spp. have shown an increase of HITH during the colder seasons and appearing to be due to an increase in Spironucleus infection rates. This has been borne out by an increase of HITH in specimens deliberately infected under laboratory conditions. There is research supporting all of the previous deficiencies I've listed as possible causes of HITH and HLLE, and there is research dismissing each of them as causes. I've never seen and currently can not find anything that links HITH or HLLE to impaired/inefficient osmoregulation due to water hardness, either via Google or Google Scholar. I'd be interested to see any paper or article supporting that hypothesis. My personal experience doesn't support it. Regardless I doubt that there is any more evidence supporting impaired osmoregulation as a cause than there is proof supporting other theories or causes. It may contribute to both of these conditions in some cases, but probably isn't a sole cause.
Most oscars don't live longer than 10 years without getting HITH or HLLE because they are poorly taken care of. Period. Because they're so commonly sold as a staple in the hobby, people don't take their status as a large cichlid seriously, or people don't understand the type of care that a large cichlid requires. Whereas other large cichlids tend to end up with specialists or at least experienced hobbyists, oscars often end up poorly fed in too small tanks with poor water conditions. They aren't treated like the big cichlids with somewhat specialized needs that they are.
WYite
