I don't know how this discussion driftet to one product, I started reading the thread, and skipped to the end....

Being a chem. engineer, and having worked years in a laboratory I can offer some advice. In a standard Paterson tank for two films, one need no more than 4 - 5 liters of water.

There are two principles at work here, dilution and diffusion.

At first one simply dilutes the fluid, this works fast, just a few seconds, but later in the process what is needed is getting the fixer salt molecules out of the emulsion, that is diffusion and takes minutes.

One also needs to take into account the rest of the last bath that remains inside the tank, every time one drains.

Dilution works all the way during the process and is what needs to be taken into account, as soon as the molecules have diffused out from film and into solution.

All this is standard operating procedure in all classical analysis work in any laboratory.

The calculation is like this : Drain fixer 600ml , in the tank remains 30ml that adheres to the tank insides, spiral, film and so on. Dilution is 30/600 = 1/20 and remains constant for every change of water.

repeated changes works this way 1/20 * 1/20 *1/20 = 0.000125 in other words concentration is reduced to 0,0125% of the original fixer bath.

In 6 changes one is down to a concentration of 0,000 000 015 or 0,015 millionth of the start.

If the procedure is something like this :

Empty fixer, fill with water and shake for 5 seconds, drain.
Repeat, shake double the time every time until this is repeated 7 times

Time increases like this : 5 - 10 - 20 - 40 - 80 - 160 - 320 seconds, no need to shake all the time but diffusion takes more and more time during the porocedure.

Last cycle will require some 5 minutes stand time, shake at first and a few times during the 10 minutes.

Concentration will now be down to 0,000 000 000 781 or a billionth of the start concentration.

One has used just 4 liters of H2O, compared to about 120 liters for an hours rinse in running water, and as any chem. engineer will tell you, even after an hour of running water the concentration will NOT be lower than after 7 changes of water.

The whole thing will take about 12 minutes, one has to work a little, the film will be perfectly rinsed and easily last 50 years as has mine since 1962, I have used this procedure for ever, except for a few years when I got a Paterson "tube", I note that there are a couple of spots on a few of the films from those years (the lazy years...) before and after they are spotless.

BTW I learned this not in school but from a german in a book : Hans Windish "Die neue Foto-Schule I, die Technik" apparently water was scarce in western germany after WW2!! The book is still recommended if it can be found, I have my copy.

Closing I will just note that archival copies on paper can be treated after similar principles. Using a couple of baths, draining the previous water out and changing 8 to 10 times should get prints that will last forever.