Do you know what a stock tank is? Its a big tank that is used for drink water for large livestock animals, but they have other uses... You can change them into a brooder which many people do.That doesn't make ant sense... you should be able to adjust it... It should have that built In function... you might need a replacement... do you still have the warranty??
They're just using a 250 watt bulb so the way you adjust it is by lowering or raising the bulb.
No replacement is needed, they just need to figure out how to keep heat in such as covering some of the top of the stock tank.