Like most of the country we woke up to very cold temps. We prepared the chickens, the rabbits, cut firewood, sealed up the house and checked on the garden but we were not prepared in the greenhouse. We had a grow light on thinking that would put off enough heat to keep everything above freezing, well we were wrong. Our main fish tank was fine, the sump and the grow bed was fine, it was the small pvc pipes in the system that froze. 
One of the pvc pipes that froze is the pvc under the bell siphon. This caused the water to back up in the upper grow bed. Luckily my husband caught it in enough time that we did not have any over flow.  He disconnected all the small pvc and brought them inside to defrost. He also used the grill as a temporary great source. We know this is not ideal but it works. In Baltimore we do not have many days with weather like this so I think a temporary solution will work for now. I think some recycled pool noodles will help insulate the small pvc pipes and more grow lights and a small heat source with a thermostat will help. We also have a part of the green house that faces south and does not get much sun. Adding ridged foam insulation for the winter would help as well. There is even insulation that has a foil backing that can reflect the light and heat from the North.
We are glad we have not lost any fish and that the broccoli that is in the system seems perfectly happy. These are lessons learned that will help us improve the system and more forward.