Well, more people said yes than no to the Bangai Cardinal idea (on other forums as well) so I went ahead and picked one up on Monday. I asked the pet store to feed the tank and all 4 of the Cardinals that were in it ATTACKED the food, so no worries there.
I took it home and drip acclimated it for just under 2 hours, got him in the tank and everything seemed fine. No aggression at all between him and either of the other fish. Later that night I fed a little bit of flake food to the tank but he would not eat any. He would just grab it and then spit it back out. I figured maybe he was just picky.
Tuesday I fed the tank some dried mysis shrimp instead and he did the same thing, wouldn't eat any. I had asked the pet store what he fed them that first day and he said brine shrimp, so I went out to the store and bought some frozen brine shrimp.
Wednesday I tried feeding the frozen brine shrimp and I think he ate some but it was hard to see cause he would grab it and then go behind the rocks, so I'm not sure if he spit it back out or not.
Thursday I came home to try the brine shrimp again, and found the Cardinal dead
Nitrate 0 (MAYBE 5, it's hard to tell)
That's pretty much perfect right? I have no idea why he died