Tetraodon mbu.
Acanthicus adonis. Pseudacanthicus histrix.
They are tank busters!
While I suggest these fish, I'm going to defend monster fish keepers, what is needed is more stringent checks and more severe penalties for animal cruelty.
I kept hypostomus plec, that's the common plec most refer to. Beautiful and peaceful.
I keep clown loaches, I have just acquired my second gibbiceps, while it isn't quite as large as hypostomus, Adonis or histrix, it is still going to be a big fish. The last one was around 38cm/15".
If I could build a tropical pond, I'd have Adonis, some rays and whatever else may fit.
If the tank fits!
http://www.fbas.co.uk/FISH CARE and LAW.pdf
But how do they enforce it, the UK law, it is common sense but at the point of sale there is not a lot to stop you you walking out with whatever you want.
People who buy fish knowing they cannot look after them for life should be criminalised, while I understand circumstances change (they did for me) the idea of growing out and selling on because you know you can't provide for the life of the fish is really something that should be bought to attention.
Better education is what we need, not persecution for a passion, some people actually must have fish, and for a few, big fish are all that matter. It isn't because of the size, it's hard to explain, while I'm a fan of things like black neon/glowlight tetra size fish, there is something about a clown loach or a big plec sp. that the small fish just can't compete with. They have more presence, grace and majesty.
People who keep arrowana, rays and red tails in the right conditions shouldn't have to suffer because of those who are blind/ignorant/uneducated/thoughtless.
Compared to some people, the fish I keep (some of which have been highlighted as non compatible) are really small!
If you have the means, have the fish! Research what you fancy, decide if you can. If you have the fish already, do the right thing, build it a big house!