Yeah, it's normal...which is good because your bacteria slow down too...and so do the plants. Most all biological activity on the planet kinda slows down when it gets cold...bears, people, trees, fungi,...AP bacteria, fish and plants...Ahhh...if only spring were eternal...sigh.
Thank you for responding.
So I take it the fish will survive
I am thinking of putting a heater in the tank
Most fish it seems can go a looong time with no or very little food, (months) and be just fine. But since you have some type of tilapia, a heater would be a good thing to think about. You may want to see with someone who is experienced in raising them as to what you can expect, at what temperatures and size out your heating system from there. Or, replace them with a more climate appropriate fish. One that will be more active (and wont croak) at lower temps, and that you wont have to break the bank heating water for them...
Here in Southern California the temps have been dropping down into the 20's. I've been running a titanium heating element (300 watt) since November. It's been on non-stop the last few weeks trying to keep the water temp up. Since I'm running a constant flood and drain, my grow beds act like giant water coolers when it's cold out. To combat that I extended the pipes that drain into the grow bed so they weren't picking up more cold air. That together with the heater keeps the water above 55. My Tilapia still hug the heater and they're eating less but they still manage to nibble here and there.
If you go with a timer you will save $ (after the timer is paid for).
My timer is set to run 5 min to fill the beds then it shuts off for an hr (your time on would depend on pump/bed volume).
Run the timer through another timer that is night off day on. Or day on night on 1 or 2 times only if you don't want your beds to have no water for that long.
For supplemental heat I use a small pump that is on a line voltage thermostat that is placed in the warmest part of my green house (up in the rafters). When the air temp is high enough the pump turns on. It pumps tank water through a black plastic hose that's up in the rafters of my green house. The water picks up just a little temp . So during the day I can solar heat the water (with sunshine). You can tune the amount of heat increase by the length of the hose. If you have a small ammt of water be careful because on the hot days you may over temp your water. This could be controlled by using an aquastat to give electric to the line voltage thermostat. Water is warm enough it breaks contact so no electric to the solar heat system because its not needed.
Those steps will save you $ allowing you to use electric on non solar times and nights.
When shopping thermostats they come in electric on or electric off with the wanted temp. So draw out your electric system before you purchase.