So...to play devil's advocate.
Yes, what she did is against Petco policy - if she ended up completely ignoring you after you left. If she did what she was supposed to, she would have informed one of her co-workers that there are dead fish in the betta section and had them removed, if she could not do so herself. If she did not, then you are free to complain to the manager all you like, however she will likely be given a warning, not fired.
Secondly, and I hate to point this out, but it happens. Most Petcos and other similar stores are likely to carry several hundred to thousand of fish on any given day. Statistically, it is nigh impossible that every single one of them will be in perfect health, and many die daily. It is a fact of life for those who work in pet stores, and many eventually just deal with it by realizing that not every creature in the store will live to see another day. So her "oh, they die all the time" may not have been quite appropriate to say to a customer, but it is, nonetheless, a fact.
Thirdly, betta cups take a long time to clean...a very long time. Like, several hours. And due to spacing issues, only one person can do them. Now, add that to aiding dozens to hundreds of customers who either fall into the three categories of kind and ignorant, obnoxious and rude, and downright nightmarish (very few customers, believe it or not, are nice people who actually understand that they are dealing with living things). Then, consider the fact that it normally takes that one woman plus 3-6 of her coworkers, and the entire shift before or after to unload that truck they got....well, there are very few hours left in the day to be spending hours cleaning betta cups. Also, I might add that there are only a few people who work in the store who usually clean the betta cups, as it is normally a duty left to the Aquatics Specialist - and I'm willing to bet that the woman you spoke to was not the Aquatics Specialist.
From the sound of it, this is normally a very well managed store who takes amazing care of their animals. You just happened to come in on a very bad day, and probably made her hectic day worse by yelling at her.
I'm not saying that the store is entirely guiltless - they probably should have had someone at least remove the dead fish, and they could have cleaned the cups the day before. However, try to see things from her side of things. Doing things like calling for her to be fired when she most likely had no part in it is silly and childish. If you still choose to call the manager, at least be more passive aggressive about it and kindly mention that they should perhaps check more carefully for dead and sick fish in the morning, before they open, when they are taking care of the snakes and hamsters.
*waits for pitchforks and torches*
Avalon - Male Butterfly Double-Tail Halfmoon, Pink/Cellophane
Camelot - Male Marble Double-Tail Halfmoon, Black/White
Merlin - Male Delta, Steel Blue/Green