Cpeckford noticed something interesting in dingers post about keeping 6-8 oscars in a 120g

It is well know that an oscar in a 20g will become stunted and lead a shorter life. Yet dinger kept 6-8 oscars in a 120g (with 8 that 15g each). So overcrowding doesnt cause stunting (assuming that you have enough filtration, do enough water changes, dont have any aggression or breeding problems then oscars the oscars will be fine). I can imagine keeping the water good would be a real effort

So why do oscars get stunted in a small aquarium but dont in a larger aquarium with the equivalent water volume for each? e.g. 1 oscar in a 20g get stunted but 5 in a 100g dont

20 gallons of water is just not enough to maintain even a reasonably small Oscar. What you've got to remember is it's not the dimensions of the aquarium that stunts the fishes growth, it is the quality of the water. If you think about it logically a 20 gallon UK tank is around 3 foot in length and about 12 inches in depth. That's not exactly going to squash an Oscar the same as if you were to wear the same pair of shoes all your life and never take them off. People who insist on keeping Oscars in very small tanks have the filtration to match. In other words, tiny little filters.

Now let's talk about a 120 gallon tank with six Oscars. Your filtration will be a lot more beefy and will handle the waste a lot better. I know that if you calculate it, the Oscar still only had 20 gallons to themselves. However, I don't think you can quite look at it like that. Filtration has everything to do with it, if you get your filtration right, you can sometimes get away with overstocked tanks. Having said all this, I'm certainly not condoning it.

So if an oscar was kept in a 20 with the right filtration and water changes it would grow to full size and not be stunted.

I am not intending to do this but i am interested to know why they get stunted.

And what about one oscar in a 100g with small filtration- would it end up stunted?

I always thought it was a combination of filtration and swimming space. The filtration need to handle the load, and a large fish needs a larger swimming space. It's just like with Angelfish, you can take a 20 gallon long tank, and a 20 gallon tall tank, and the angels will do better in the tall tank. Different fish need different enviroments.

Stunting of growth is not caused by lack of filtration. Stunting is actually caused by the fish themselves. Most animals on this planet have the ability to give of tiny chemical messages called pheremones. They use them in all different ways the most popular is to attract the opposite sex. With fish they are able to give off a pheremone to stunt the growth of the other fish around them thus ensuring that the fishes survival rates are higher. The faster the fish grows the more chance it has of dominating the others. This means that when you have lots of fish growing on together then you have lots of growth stunting pheremones (dont try to say that when your drunk) in the water. The only way to get rid of them is to carry out water changes with fresh water in order to dilute the pheremones. In the far east where they farm fish commercially they carry out near 100% water changes everyday.Each day the tanks are drained down until there is just enough water in the tank for the fish to lay sideways in and then they top them back up with fresh water. This helps them achieve phenomenal growth rates so that they can get them to sale size quickly and economically. So its not the size of the tank that governs the growth of the fish but how often the water is changed.

Now i understand. So one oscar kept on its own in a too small tank stunts itself

One more question, if you do enough water changes to keep nitrates to acceptable levels will this be enough to stop stunting?

Well it should be but im not sure how big an Oscar would grow in a 10 gallon tank. I suppose it would be great to see an experiment where the size of the tank and water changes were different but would take up a lot of time. There is also the issue of whether it is fair to have a large Oscar in a smaller tank.

You are thinking about this wrong, if you have 5 oscars in a 100 gallon tank they don't each have 20 gallons they each still have close to 100 gallons to swim in. One oscar does not displace 20 gallons of water, but 1 oscar in a 20 gallon tank only has 20 gallons available to it. I hope this makes sense to more then just me.