How many gallons should you put 6 oscars?

Read the tank size article on the site. It will give you an idea. If I had to give you a "ball park" estimate it probably have to be 200 gallons or more. Also, excellent filtration is a must for just one oscar so when accounting for six of them the filters would have to filter through the aquarium more than just 5 times an hour. Hopefully someone can give you a more definite answer.

If I were looking at 6 Oscars, I wouldn't go any smaller than 300 Gallons.. With a sump for maximum bio capacity. with a 5 times per hour turn over rate.

in a fish farming info, the water requirements should be at least 21 liters
of water for every inch length of a fish. if you have 6 fully grown oscar at
10 inches each, you will be needing at least 327 gal tank.