Why did my gold algae eater die in a tank full of betas and why did one of my betas die the day after?

Answer 1

If you go to Petco and look at the chart of what fish can be paired, you'll easily see that the Betta fish are incredibly aggressive. Just because they seem docile, doesn't mean they are. Why do you think they are quartered in separate containers?

I would be cautious about what type of algae eater you get because depending on the type, they can grow up to be quite aggressive themselves. Plecostomus, sometimes commonly called "plecos," can grow up to 12" long quite quickly and will eat almost anything in the tank they think they can reasonably eat--- that might include your fish.

Bottom feeding fish like Cory catfish usually stay quite small, are more docile, and do quite well in groups; unlike the pleco which is quite territorial and should be the only bottom feeder.

An algae eating fish is, as always, not a substitute for regular aquatic maintenance. You must also feed your algae eater by purchasing bottom feeder wafers, as you cannot expect them to survive on scavenge alone. That might be what killed your fish, not the bettas.