I'm sorry, I didn't mean Petra or Tetra. I meant Platy. I got a Red Wag Platy, and a Dalmatian Molly. So, is the Platy at least a little more suitable than the Molly?
What happens if I try to keep them...I mean, I guess they die, but how long? Surely they'll have the same fate if I take them back to Petco. Those tanks are pretty small and have loads of fish in them, a lot of them eating their dead friends.
Mollies should be in a 20g
Platies in at least a 10g
A 5g besides a Betta,
Other choices, NOT WITH a Betta... Chili Rasboras, or A Scarlet Badis with Cherry Shrimp.
But true Nano fish are hard to find at LFSs.
If your Petco has a lot of dead fish try to find another store.
Not all Petcos or Petsmarts are the same. Some are better. Some LFSs are great, some are pits.
Petcos tanks look small, but they are part of a huge system. Think of a 900g tank with dividers and a massive filter. When it's maintained and they have a good supplier it's actually a decent system. There are UV Sterilizers running as well, if that store keeps the bulbs replaced.
I worked at a "good " Petsmart. Our tanks were clean, filter ran well, I made them replace UV bulbs. We would get sick fish in sometimes. I would warn people to avoid buying out of those tanks, if it was Ich or something contagious. Losing a fish or two from stress happens.
Learn what a healthy fish looks like. Clean fin edges, good weight, no scratching or spots.
Your Petco sounds pretty bad, unless they just got fish in and hadn't had a chance to pull dead ones.
Mollies are almost a Semi Aggressive fish. They often bully and harass other fish. Esp in smaller tanks.
I would return both fish unless you just want one Platy and nothing else. It's not ideal.