a single oscar needs a bare minimum of 75 gallons, about 120 for 2. spotted gars get about 3 feet long and need a tank that is at least 3 feet wide and probably 9+ feet long.
fire eels get about 3 feet long and shouldn't be kept in anything smaller than a 100g tank.
someday i plan on a 180g tank with a tiger oscar, 2 fire eels, and a trinidad pleco.