I recommend feeding them only raw food. This actually costs far less than feeding premium kibble. It will keep them out of the vet, thus saving you money. This makes the skin smell and taste acidic to the parasite and will repel them. It repels fleas, ticks and flies.
Parasites attack ill and unhealthy animals.
Use the real thing even though it is slightly more expensive. I use just a few shakes, maybe adds up to a teaspoon. Add it to the water while they are getting used to the smell and taste and if you continue to feed kibble. I am an ex-exterminator. I don't believe many pests truly need professional treatment.