I have 2 ACF in a 20 gallon long tank. They've been in there for 8 years!
There are pond stones on the bottom large enough that they can't swallow them. They hate the tank light so I leave it off. The filter is off too because they slam into the lid when the tank is filled deep enough for the filter. (I'm considering under gravel filtration but don't have time or money at the moment.)
In my experience they kill and eat all tank mates, even ones larger than their size!
They also rip out any plants and destroy them! I have fake plants in there and the plants stay floating around the tank. Every time I anchor them, they tear them back out.
Basically ACF operate under the "kill and destroy" lifestyle, haha. They need a few good hiding places and that's about it.
On the other hand they are extremely hardy and seem to survive anything, including horrific water conditions, being eaten by my cat for an hour, being lost outside of the tank for 2 weeks, and forgetting to feed them for awhile.
I got mine in college not knowing what they were exactly and they just never die no matter how badly I cared for them!
We love watching ours swim and play. My 3 year old especially enjoys them.
Mine eat a diet of random bugs we find, reptile sticks, freeze dried blood worms, sinking shrimp pellets, and sometimes a bit of fish food.