You should try to enclose your setup in a closed building or greenhouse of some kind... you can and should insulate the sides/bottoms of your tanks/tubing and (partially) cover the tops... you can also add additional "thermal mass" (add more water tanks) to help slow down the loss of temperature. It's all about conserving the heat in the system and reducing the rate at which it radiates/conducts/convects out into the environment.
As for adding heat: Search out tips and videos on in-line water heating, rocket mass heaters...
If you enclose with a greenhouse, be sure that surfaces are black, if possible, (use black materials or paint the dry areas of your setup black) to help absorb solar heat and re-radiate it inside the greenhouse.
Be cautious with making these changes too permanent, though... during warmer months, if your temperatures climb into the 30 (C) or 90 (F) range, you'll need to find ways to ventilate and reject heat to keep your fish/plants cooler
Have you considered one of these?
There is no need to actually run the fish tank water through the bin, you may consider running the water pipes like an element under the fish tank and warm the water from the bottom of the fish tank and let the heat rise through the water.
With a good lid, you may even be able to heat the air above the water to assist and reduce the exchange above the water.
Just a thought.
I agree with the comments others have sent you. I played around with multiple aquarium heaters in my IBC tank and finally built a heater I found on YouTube. The only change I would make if I did it again is to go with a smaller element because of the extra electrical load the 2000 watt element creates. I Followed the first video below but there are other options. The most important and expensive part is the controller. I used a Ranco ETC-111000 controller I got on Amazon. The sustained heat it provides definitely helps het my little greenhouse.
I guess we can do a lot by improving the design of our systems. For instance, our fish tanks or channels can take advantage of "thermal expansion" by being wider or with more surface area below and narrow or with less surface area exposed to the cold. My sister in Maine is experimenting with a buried fish channel that is 6 feet deep and 4 feet wide at the bottom while only 1 foot wide at the top. In a bottle neck upper shape popular in Stanley thermos bottles.