I recently finished a piece for a European fish magazine that discussed marine parasites and modern vs. old treatments.
I found I spent a lot of time on hypo salinity techniques. This is an invaluable tool for the marine hobbyist as it does little harm to the system yet it makes life very difficult for life stages of common parasitic species that plague many a marine system. It is estimated that more people leave the marine hobby in their second year due to the looses attributed to parasites that cause 100% wipe outs in their aquariums.
For you koi folks, the marine hobby has ‘KHV level wipe outs’ constantly. 100%. Imagine? We have only experienced that kind of catastrophic reality twice in the US koi hobby experience—once due to aeromonas and once due to virulent forms of KHV. The marine hobby, on the other hand sees 100% wipe outs every day in America.

The common thread in the aeromonas wipe outs (once upon a time it was 80%) and the marine parasites’ 100% wipe outs (oodinium and related protozoa) is the parasite vector. Indeed most of the wipe outs in koi ponds in the late 1980s were due to the presence of protozoa like trichodina and higher forms like flukes.
It was AKCA to the rescue in 1989 with its collection of data and shared members experience that revealed that aeromonas was a dynamic and not just a bacterial ulcer. This revelation gave rise to the creeping trend in HypERsalinity treatments that we see today in ponder advice.

Well like in the ‘Opposite world’, the marine hobby pursues the same trends but in hypo salinity—that is, the reduction of salt level concentrations in water.

In both cases, freshwater and saltwater, the idea is to disrupt and outright kill lower forms of life that can’t cope with osmotic gradient ranges beyond what they are evolved to.

But here is the rub--- MANY of these species from oodinium that lives in brackish water, to Asian flukes and trichodina that live in brackish waters, not all species can be killed with simple osmotic disruption. Indeed some can hang in almost as long as the fish can!

So hypER or hypO salinity is a limited option in that it works—until it doesn’t!

In the case of koi ponds, we have moved the needle higher and higher in order to keep up with ‘resistant’ species of parasites. They are not so much ‘resistant’ as they are locally grown to adapt to their natural conditions over a life time and over many generations. Many ponds in south Asia, for instance, have salted fresh water as their source water. This is what makes some of these fish hard to acclimate and their associated parasites practically indestructible in terms of salt treatments ( salt 'up' or salt 'down').
What is most effective in the marine hobby is to use hypo salinity technique with a killing agent—a one/two punch. The same can be true of freshwater. But in freshwater the doses have to be less in the presence of hyper salinity.