One is water change schedule, and because it may be the same now, as it was 3 years ago, that cute little 3” oscar is quadruple that size, and putting out quadruple the waste, but those 50% water changes might be still only once per week.
Results the fish is swimming in fish pee, and fish poop soup, and high nitrates (no matter how great the filtration).
When most people get their tap water, it is 99.999% pathogen free, and close to nitrate free.
As soon as it is dechlorinated, it becomes a great home for nasty bacteria, and the more your oscar pees and poops in the water, the better place it is, for those bacteria to grow, so even with a 50% water change there are 6 days for bacteria to double, triple or more. Many pathogens reproduce at alarming rates.
I've always believed that this is one of the key stressors in Oscar tanks, and it is why many people still feel that HITH can be cured with nothing more than water changes. Sometimes, yes, other times not so much.
While not all black water species are susceptible to HITH, or even all species of fish, among some of the cichlid species HITH is definitely an issue. Oscars being an extremely popular species, along with probably one of the most abused cichlids kept in captivity, it seems we see or hear a lot more about HITH in Oscars, than other cichlids. Like Duane I have avoided certain species over the years because I have seen first hand how sensitive they can be when kept in my local water. I don't need to be a chemist, or expert on anything, they eyes don't lie. Those fish require near pristine conditions to thrive. Clean water, clean filter media, and an overall low stress environment. Those species tend to develop chronic HITH if anything is off, and with some individuals it doesn't take much. I made a post on this subject a few yrs back and it was later made into a sticky. Part of that sticky …..
Over the years numerous things have been blamed as the cause of Hole in the Head disease in fish. While this is still a confusing subject among hobbyists one thing has become clear, the pathogen responsible for the ulcerated pits in the head and body of ornamental freshwater fish with this...
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Over the years numerous things have been blamed as the cause of Hole in the Head disease in fish. While this is still a confusing subject among hobbyists one thing has become clear, the pathogen responsible for the ulcerated pits in the head and body of ornamental freshwater fish with this condition, is Spironucleus spp., in ornamental warm water fish, the culprit is typically Spironucleus vortens.
Water quality can often be a stress trigger among many species of fish, with excessive stress creating a lowered immune response which in turn opens the door for protozoa to proliferate to excessive numbers. No matter the stress trigger in each individual case, it is always some form of stress that triggers a case of HITH. Poor water quality, poor diet, stress from aggressive tank mates, overcrowding, etc, can all lead to an outbreak of S. vortens flagellates. With any type of illness, be it bacterial, viral, or parasitic, pristine water quality will help ensure a heightened immune response by the fish.