Here's my input:

1. Scylla LOVED tilapia. I just read in this, Amazon.com: Quick and Easy Ribbon Snake Guidebook, that tilapia itself and other fish is ok as long as you supplement it w/ reptile vitamins like, once a week or so. The snake will get plenty of protein since it's essentially just flesh & muscle, so you'll have to supplement for the minerals missing from not digesting any bones and organs.

2. Trader Joe's is FAR more reliable in their merchandise than Wal-Mart. As long as you buy the fish frozen, I'm not seeing a problem with parasites, whether it's tilapia, salmon, or anything else. If you're worried about mercury, pollutants, and other man-made menaces, I think you'd have to leave the planet to escape it all.

VARY the diet as much as you can based on your snakes known preferences for food. I'm not entirely sure that what you feed it specifically is as important as how much variety there is, if you follow me.

Shop around for the book i mentioned too. I got it for like, $10 from a seller on Amazon, but shop around for it. The seller I got mine from is out of stock, and other people are asking frakking ridiculous (and I mean ridiculous) prices for it.