A few things to think about..
Overcrowding supresses aggression, I've done it before, as well as others. If done properly, it should cause little to no stress on the fish, and they should live fine and well.. If a fish can't claim any territory, it will often become a 'defensive' fish, instead of an 'offensive' fish.. Thus, the entire tank has the mindset of 'Ok, lets all play nice..', unlike someone trying to stick 2 oscars in a 90g tank.
The tank is likely around a 1000g, and is probably twice the water volume if you add into filtration. Keeping a barebottom tank clean is not hard.. Especially if you have a water movement to move fish waste from the bottom of the tank, all to one corner, so it can easily be cleaned. I also imagine this person has an automatic water change system plumbed into the tank.
They weren't in different tanks, and placed their for the photo.. If this was the case, there is a good chance the fish would be dashing around, or the oscars would be locking jaws.
and last, but not least..
The far east monster fish keepers, are the ORIGINAL, and creators of the concept of 'Monster Fish Keeping'.. and they have had probably twice as much time and experience as the US counterparts, thus making their methods and techniques likely more advanced than ours. When we were screwing around with Discus and African cichlids in the 70s, they were designing filtration systems and tanks for Arowanas, Oscars, Giant Gouramis, Labeos, Goldfish and Koi..
I wouldn't knock on something that is that badass.. It's for display purposes, it's not like he is trying to build a bio-tope or breed fish.
