It all depends on where and who you ask. In Asia most people will say 2' tank is ok for Arowana; in the West most would say that's too small and that at least 3' is best for Asian Arowana, most of which very rarely get to a full 3', most are closer to about 24". Silver Arowana on the other hand can get even longer, so most people would say 4' wide is eventually required (at adult size).
For Arowana and many other monster fish it is recommended that the width be at least 1.5 times the length of the fish. Think about it: if you were a 2' fish in a 2' tank you'd barely be able to turn around. Putting it on a smaller scale, would you keep a 3" fish in a 3" wide tank? Some people are ok to scrunch fish into tanks, I personally am not. I have 2 Asian Arowana in 2 X 325g tanks that are 84X32X28. The fish are currently about 15" & 17". In a year or two it's likely I will need to upgrade to bigger (esp. wider) tanks.
For Dats a 2' tank would be fine for quite a while (they're very slow growers compared to most other fish) -- but as I said above, my preference would be for something wider when it got to adult size. The more room you give your fish the happier and healthier they will be.
I think this is a great answer and perspective. I share Islandguy's view about preferring to keep fish in larger than needed tanks and preferably ahead of schedule/before it's needed.
To your other question, Indos can potentially reach +24" though in the wild this seems somewhat rare (in part to fishing I would guess) and even more so in captivity for a variety of reasons, the main ones being slower growth rate. As datnoids age they tend to grow slower as time moves on and size increases. Another reason is that many people I've known grow tired of them before they reach mature/monster sizes. Truth be told datnoids have very unique behavioral traits and they aren't for everyone, they don't gave the color or vibrant personality of most SA/CA cichlids or the stoic, predatory looks of many catfish. They are what they are, sometimes stable sometimes full on brown, ambush predators who act like it. Oh, and they basically don't breed. It's not too surprising they aren't the most popular fish outside of the sort of niche group of dat keepers.
Unfortunately because of this and slow growth I've found many just don't get the proper care, housing, tankmates, attention to grow and prosper into true monsters that put big 12" fish to shame and because there tend to not be many big ones on display frequently people sometimes neglect that they grow as long or longer than the width of a standard 180g tank.
That said a 180g is adequate for at the very least a few years though longterm once you get close to 14"-16" you would want to look into something at least 36" in width to be ahead of the game and keep healthy growth rates and happy fish moving along.
Hope this helps.
The-Almighty-Zugs
see it like this lets say you are 60cm wide over your shoulders would you feel confy in a room 65cm wide? Same goes with fish. Bigger is always better. With that saud id say you can have an indo in an 6x2x2 tank for some time. We are talking some years but i would not keep an indo in a tank that size for life. Im currently growing out an indo in a tank thats 200x60x50cm and the indo is about 35cm now and i feel the tank is getting to small for the fish. But i cant put it in with my pack of cichla orinociensis yet as they are hotheaded and pick on everything that can defend itself.
