In Southern Cali. I never dreamed that temp would be the problem. A year with a functioning system, cycled well, plants extreemely productive , even had 5 females spawn. In January 1800 talipia were floatuing belly-up, the temp was 48 degrees. My tank sitting directly on the ground, just got too cold. I managed to save 30 fish ranging is size from an inch to 3 lbs. No amount of heat from the conventional immersion aquarium heaters made the slightest differance. I move the fish to a protected tank indoors, placed on a 2inch thick pad of styrofoam type of insulation, this allowed the immersion heaters( 400 watts x 2) to elevate the water to 80 degees. I am now building a rocket stove next to my outdoor system expecting to produce about 400,000 btu's of heat at "full run", Outdoor temp is still ranging from 70 degrees midday to a low of 43 degrees at night. In my moderate climate inorder to raise edible fish versus ornimentals an unanticipated input into my system is HEAT.

Good luck

Elizabeth I am having the same problem with water temp - course it's starting to warm up now, but I tried a lot of things over the winter with minimal success.  Do you have your tanks in ground or above ground?

Welcome to the forum Elizabeth.