Thats cool, so the 3x 300 heaters and insulation kept them @ temp? Forgive my ignorance but what is plc controlled heating. I know we had quite a few hits below zero my way and I am glad I have a huge basement.
PLC stands for programmable logic controller which basically is a separate controller that you attach a thermal coupling to and using a relay to control the heater. The one I have you can program for a set temp which has a .2 degree tolerance or give it an operating range. It has an alarm feature that I will be using to control the fans. When the water gets too warm which it will in the summer months the alarm will kick on a fan to cool the water. If the water gets too cold then it will kick the heaters back on.
To answer your other question yes 3 x 300 watt heaters kept my previous 240 gallon pond warm. I built it out of 2 x 4s and used R13 insulation all around and covered with blue foam board that had a piece of plexi on top so the light could shine through.
I did not do it straight on the ground because if the floor gets wet beings it is in the garage I did not want water to creep under it and not be able to dry it up. I am going to fiber glass all the joints then use either sweetwater epoxy or pond armor. Not sure which one yet, leaning more towards pond armor