Which fish should not be with others?

Honestly any fish that isn’t of the same family or type. For example people love to put Angelfish and Neon Tetras, but in the wild the neons are food for the angels.

Similarly people mix in cichlids from different lakes or even add them in with more peaceful/community fish and wonder why fish keep disappearing sadly. A big one that is common is where people will put in a tank multiple bettas (it can be done for a sorority), but I’ve seen like a 5 gallon with 3 bettas which is far too few in a small space to work out nicely.

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Most obvious ones include bigger fish that are with smaller fish that can fit in their mouth since they will be food in a heart beat. Additional cichlids of different lakes or areas, or those of different temperament.

I would say make sure you always watch out from cichlids (African do not mix with Central/South American), and you can also run into issues from community fish with other semi-aggressive fish that may look small but are a terror

I would not put more than one beta in a tank both because they need a lot more space than people realize and because they can be very aggressive with each other (which is why some people think it is amusing to hold mirrors up to them and watch as they try to attack their own reflections).