If you are planning to keep Oscars then you really must provide them with an aquarium that is set up specifically to cater for their requirements. We always recommend going for the absolute largest tank you can afford, or house into your property. Many people seem to agree that 55 gallons is about the minimum size aquarium if you want to keep one adult Oscar. I would actually recommend no less than 75 gallons for one adult Oscar. The extra room will really benefit your fish and will significantly reduce the amount of maintenance needed if accompanied with a good filtration system. Once you start thinking about more than one Oscar then you are looking at tanks in excess of 100 gallons.