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Don't be too complacent, you haven't got long to sort a new tank out. Oscars grow at an alarming rate. You already know that your tank is too small at the moment. The absolute minimum is 55 gallons for one Oscar on its own. However, my recommendations would be 75 gallons. That fear extra gallons will make a difference, believe me. As for how much it will cost, why not visit your aquatic centre and see what they've got. You can always buy secondhand although you've got to be a little careful. Try and view a tank full of water before you purchase it. You never know whether an empty one has got a leak.