Ah!!! I got it! Duh,...
In fish rearing systems, the main problems is bacterial buildup and fungus, besides the obvious - nitrates and phosphates.
What he is doing is taking out all of the clownfish in a tank and leaving it completely barren, (these vats could be several hundred gallons at a time) and then chlorinating it to kill bacteria, fungus, viruses, etc., then running the water through a filter to get rid of the solids, probably running it through a large skimmer to get rid of DOCs and then dechlorinating it to reuse the saltwater.
In fish only systems, the saltwater just needs to have a certain amount of basic elements for the clowns to grow and thrive - calcium, magnesium, alkalinity, strontium, iodine, boron,... He is re-using the water so that he can cut down on the cost of salt. While this may work for him, I really don't recommend it unless you don't have the means to buy salt. I also don't recommend it for tanks with corals.