Not necessarily a parasite, it could very well be constipation. If they are hiding, pointing downward at about 40 to 45 degrees, pectoral fins clamped to the side, possibly breathing harder, and slightly darker in color and not shedding white poop it is in fact likely constipation.
It's also not how often you feed them, it's what they are being fed. If the first ingredient is fish meal, throw it out and go get something like Omega one.
Start with about 1/2 Tablespoon of Epsom Salt per 10 gallons of water. It may help them out pretty quickly. Repeat with another 1/2 Tablespoon per ten gallon in four hours if you do not see immediate improvement.