We've taken a number of steps this winter to assure the health of the fish in the system. Now, I must admit, we seldom get this much snow in winter but it certainly makes the point.
Reducing the effective area
Our greenhouse has a 15 foot peak as part of the solar design. There are crossbeams at ~8 feet above the floor of the greenhouse. We put a layer of clear roof panels (polycarbonate) over the crossbeams to effectively create an upper and lower area in the greenhouse.
We do notice substantial temperature differences between the two sections. At night with outside temps in the 30's and 40's, the bottom will average 10 degrees warmer than the top of the greenhouse. On the coldest nights with the outdoor temperatures in the teens and the heater @ 1500 watts, the polycarbonate barrier creates a 20 + degree differential. The polycarbonate panels were a recent addition this fall and we notice a big difference in retaining heat. We may choose to try them through the summer and hope for a more moderating effect.
We used 3" PVC pipe that runs from the top peak of the greenhouse to the floor. There is a hole cut in the roof panel layer at the midpoint for the chimney. At the floor, the chimney has an elbow and turns to point at buckets of water under the growbeds. These buckets are part of our heat sink for the greenhouse. There is an axial fan in the pipe at the floor level that pulls air down from the peak of the greenhouse. This fan is hooked up to a thermostat so it only runs when the temperature in the top is above 70 degrees F.
Solar collector water heater
We used 3/4 inch SIL-O-FLEX® Polyethylene Coil Pipe to create a solar heat collector. The product is potable water grade and PE is typically good to 120 degrees F in low-pressure applications and to 200 degrees in non-pressure applications. The pipe will be covered in summer time to keep from overheating.
Water is pumped up from the bottom of the fish tank by a separate Pondmaster 950 pump through the pipe into the upper area where the pipe is spread out to collect solar heat during the day. There is 180' of pipe up above. So far, we've noticed about a 3-4 degree (F) increase in temp when this runs. Note, we are in an area where we can go for days with no sun and then it might peak out through the clouds for an hour or two. This hour or two is when we see some heat gain. The pump is run by the same thermostat as the solar chimney fan.
Covering fish tanks
We used a piece of acrylic to cover each of the fish tanks. This could be done more cheaply with styrofoam or foil backed foam insulation panels. I'd recommend taping the outer edges of the styrofoam to prevent bits getting into the tank. The acrylic is actually heavy and a bit cumbersome to handle.
Turn off fingerling tank
This summer we added a 100 gallon stock tank into the system. It sits over the main fish tank on steel supports. It is plumbed in to receive water from the lower fish tank and water dumps back into the fish tank at the same rate.
Since we don't have fingerlings yet, we turned off the water cycling and are using it to hold extra water for the system. We noted the water temperature was consistently cooler in the tank since it is not insulated. A small 10 watt pump circulates water within the tank to keep it oxygenated.
In the future, if we have fingerlings going into winter, we'll need to find a way to insulate the tank to prevent heat loss. One possibility would be to construct a box from the acrylic we currently have and insulate in between.
Oil filled electric heater
We use an oil filled radiator style electric heater for heating the greenhouse. This is the third winter with this heater and it seems to hold up to the humidity of the greenhouse. I also like the options of 600, 900 or 1500 watts for heating. The heater is hooked up to a thermostat so it only goes on when the temperature in the greenhouse drops below 65 degrees F.
Fish tank heaters
We have 4 - 300 watt aquarium heaters in the fish tank itself that are set for 65 degrees F. We built a small square out of pieces of PVC and 4 PVC elbows to create a float to tie the aquarium heaters to. This is fastened to another piece of pipe to keep the heaters in an area next to where the water flows in from the growbeds, keeping constant circulation going around the heaters.
Is it effective?
As to effectiveness, we have been in the midst of an artic chill in the Pacific Northwest. Nighttime temperatures have fallen into the teens and daytime temperatures have been in the 20's. The oil filled heater was turned to 1500 watts from our "normal" setting of 900 watts. Otherwise, the water temperature has stayed between 62 and 64 degrees F throughout the storm. The greenhouse temperatures have stayed in the upper 50's. The combination of factors has been sufficient to keep the tilapia in the fish tank healthy.