They are expensive but available, about $650 for one geared to a 180g tank through big Al's. You can make one that will do about a 30 degree drawdown for a 300g by getting a 2-2 1/2 cubic foot refrigerator (under $100), a 1250 gph inline water pump (about $70), and lots of 1/2" plastic tubing. Drill two holes in the fridge big enough to admit the tubing and coil most of it inside the fridge, silicone the tubing at the holes, the inline pump to the side of the fridge and use it to pump water through the tube. the drawdown can be adjusted with the fridge's thermostat and by adjusting the ammount of tubing inside the fridge. It does not wreck the fridge as long as you make sure you do not strike a cooling tube when drilling so if you no longer need a chiller just remove the tubing and plug the holes. The one I helped make had small freezer section that my friend used to store frozen fish food and still had room to keep a few beers in.