One of the disadvantages to buying feeder fish from a pet store is that the pet store's multi tank filtration system mixes water from different tanks and increases disease communication. When you buy feeders from the pet store you are taking a risk whether or not it carries a pathogen that can infect your pike.
There are two ways to prevent feeder fish from spreading disease to your pike.
1. Get a quarantine tank and quarantine new feeders to make sure they're healthy.
2. Breed your own feeder fish.
Livebearing fish are very easy to breed and make excellent feeders. I myself breed guppies and often find deformed, defective fry that should be culled to maintain the straight spines and coloration of the guppy strain. Deformed fish need to be removed from the colony in order to maintain its quality. A large enough guppy colony could produce enough cull fish to feed to your pike. Also, a well culled high quality guppy line is desirable on its own. Really beautiful guppies command a high price on websites like aquabid and ebay. You're not limited only to guppies; platies, swordtails, mollies, mosquito fish, Heterandria formosa, etc all make excellent breeding colonies. Any easy to breed fish would work. With livebearers you can feed mostly the males to the pike, keeping the females around to produce offspring. For example a colony of 50 fish that is 15 males and 35 females would produce many more young than a colony that is 25 males and 25 females. This allows you to choose prime stud males, too, which is important for the gender dimorphic fish like guppies.
Anyway, I'm blabbing on now, but yes, there are downsides to buying feeder fish from the pet store. And there are upsides to breeding your own high quality colony of feeder fish. Here is one way to do it:
1. Browse aquabid.com for a pretty livebearing fish you like.
2. Find one that's selling for (and costs) a fair amount of money. Avoid $1 fish.
Yes it is a larger initial investment but later when you're selling these fish you'll be glad you got ones that sell for more.
3. Order a pair/trio/reverse trio/whatever. It's a good idea to get not just one but two similar strains and cross them so that your fish aren't all inbred and nasty. Also, creating your own strain prevents plagiarism problems. If you bought someone else's fish and then started selling fish that looked 100% identical to theirs they could come to dislike you. Worse, they could compete with you, lowering your fish's selling price. If you sell your own strain and the fish look unique, you can charge more.
4. Allow them to breed in your breeding colony tank. A 55 gallon or larger works well.
5. Once they get around 50 fish or so, start picking off the ugliest ones and giving those to your pike. Give more males to the pike than females.
6. Pick a nice trio or pair from your colony, put them in a temporary holding tank, take pictures, post the pictures on aquabid.com, and tell people these are the fish they're bidding on, these exact ones. If you crossed two similar fancy strains like I suggested, your own fish will look unique and the picture will capture people's attention. Fans of both strains will most likely flip out that you've done that cross and hurry to bid on your fish.
7. You earn lots of money, your pike eats healthy food.