There is a reason not all fish in a tank where ich is running wild will come down with it. If a fish has had and beaten ich, it often acquires an immunity. This is not a long term permanent immunity, but how short is not known. When it comes to research on most things fish, it is the aquaculture industry that is most involved, not the aquarium industry.
Most aquaculture involves food fish. The regulations regarding what medications and treatments are allowed relate to their safety in terms of what fish we will end up eating. A lot of what we in the hobby may use to treat ich and which will work fairly well are not legal to use on the food fish trade. And when it is not human health related, some meds are prohibited because they will harm the environment.
Not only do some fish which survive a bout of ich aqcuired immunity, but some species are more naturally resistant to it.There is, of course, science on all of this. I have just spent about 90 minutes searching for papers on ich becoming resistant to some ot the known most effective drugs. I have had no luck so far. What I did find was a lot of research into fish becoming immune and on the search for a vaccine which will prevent ich. The theory is that if fish can develop an immunity, There must be a way to bring this to play in a vaccine. However the search is still ongoing as science has not found a vaccine so far.
So I would ask the loach if he can provide any links to the science regarding which of the more effective ich treaments to which Ich has developed a resistance. I would like to add such info to my bookmark reference list for fish meds and diseases.The odd part is when I start to search for ich having acquired drugs resistance, the results are all about how fish have acquired an mmunity ich. I will keep looking.
For those who like the science, here is another good paper on ich.
Louise von Gersdorff Jørgensen,
The fish parasite Ichthyophthirius multifiliis – Host immunology, vaccines and novel treatments,
Fish & Shellfish Immunology,
Volume 67,
2017,
Pages 586-595,
ISSN 1050-4648,
{url{
http://www.sciencedirect.com/science/article/pii/S1050464817303674[/URL]
Abstract:
Ichthyophthirius multifiliis, the causative agent of white spot disease (ichthyophthiriasis) is a major burden for fish farmers and aquarists globally. The parasite infects the skin and the gills of freshwater fish, which may acquire a protective adaptive immune response against this disease, making vaccine strategies feasible. However, there is no prophylactic treatment available and repetitive treatments with auxiliary substances are needed to control the infection. Historically, a variety of drugs and chemicals have been used to combat the disease but due to changing regulations and recognition of carcinogenic and environmentally damaging effects the most efficient compounds are prohibited. A continuous search for novel substances, which are highly effective against the parasites and harmless for the fish is ongoing. These compounds should be environmentally friendly and cost-effective. This review presents recent progress within host immunology, vaccinology and a description of novel substances, which have been tested as treatments against ichthyophthiriasis.