Wasting Sickness and Constipation

In searching for information as to what was going on with my tiger barbs, I was put onto the idea that it might be wasting sickness by a more experienced fishkeeper. He suggested that I try FLUBENDAZOLE, which some fishkeepers have found useful against wasting sickness. I ordered some from a doctor, who was kind enough to share some information with me about the nature of the illness and ways it could be treated and prevented. I thought I'd share that here, and ask for further suggestions.

He wrote me, when I told him about two fish that currently are succumbing:

Actually, depending on how far gone the sick fish are, the medication could help. The best prevention for wasting disease is to keep food moving through the gut. This keeps the Protozoa from settling in and
developing a problem. The bacteria which cause the irritation provide
a place for the Protozoa to get nourishment. Keeping the food moving
through keeps the gut clean. The Flubendazole will kill off a
flourish of Protozoa but it really doesn't eliminate them all
together. Good tank maintenance and fresh water is the best track to
follow.

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Now I change 50% of the water at least once a week and vaccum the gravel, and my levels are good. I am stepping up the water changes, but I'm thinking that the problem here might be related to the digestive system of my fish. To put it frankly--they eat like pigs but they hardly ever poo. Hence, I think that the poor digestion might have alowed the protozoa to take hold.

I feed the fish peas twice a week, and vary their diet with protien and veggie based foods. However, still no pronounced poo. In fact I almost never see them poop--so as the stuff their mouths, they don't poo and they waste away. I'm trying epsom salts today, to see if that helps them free up their digestion. What else might I do to better their digestion following the treatment, which I will begin when I get the meds on Saturday?

I sit next to the tank almost all day almost everyday--and these are active fish--plus the gravel vac doesn't bring up much poop.