Generally puffer fish should be kept in a single species tank because they will eat anything and regularly attack and eat the other fish in the tank. Puffer fish can also release toxic chemicals into the water when stressed and these chemicals will poison everything in the tank. If you have the fish in a single species tank and they do poison the water, you lose the puffers but nobody else dies.
If you keep puffer fish you should have Activated carbon in the filter and replace it every couple of weeks. If the fish does release poison into the water, the carbon will hopefully adsorb most of it and stop the fish dying.
If you ever see the puffer acting strangely or looking unwell, do a 75% water change and gravel clean the substrate immediately.
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You mention getting a flowerhorn. These are pretty big cichlids that should not be housed with puffers or tetras because they kill and eat tetras and stress the puffers, which in turn release poison and everyone dies.
If you want a flowerhorn cichlid, make sure it has a big tank with suitable tank mates.
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There are a few brackish water catfish found all around the world, but most grow too big for aquariums and are not sold in pet shops.
Don't put catfish with puffers because it never ends well.