Technically you do need a PALS card to get any sale price on anything at Petco, just like at Kroger, etc.... though if you don't have one, a lot of times the cashier will pull up a random card anyway, so if you got the sale price without having the card, that's probably why. (Like most stores that have "discount" cards, they keep track of what percentage of a cashier's customers use the card, and they can get in trouble for having low percentages, so that's why they do that sometimes.)
Card's free, though, so it's no big deal to get one. They send out some pretty good coupons... 10% off your entire purchase is pretty common. If you shop at Petco a lot, it's worth it.
Hmm, maybe I can get that 20L I've been eyeing... of course, I'd need a stand for it, and the nice ones are never cheap enough for me. Maybe I'll peek around on Craigslist or hit up some yard sales to see if I can find one for it.