Wow you're not kidding about the prices. Frozen fish sounds better than that cost.
any reason one of these wouldn't work... I just got a 100 gallon stock tank and this is the solution to keep tanks de-iced for livestock. http://www.tractorsupply.com/en/store/allied-precision-2002dp-unive... and they are cheap comparatively
Jonathan, It is thermostatically controlled. Since it is sold as a de-icer it will most likely not warm the water hot enough to be any use in most aquaponic systems.
Ryan, How many gallons does your system have all together, counting tanks, pipes, pumps, and grow beds etc... ?
What type of fish? Trout require less heat and thus a different thermostat design than one for use with a higher heat design for tilapia as an example.
Is your system completely out doors, indoors, or a combination?
How much do you expect to have to spend?
Would you consider buying parts and hiring a qualified electrician to install it?
There are heaters for a wide range of applications that you can buy through industrial systems suppliers. But generally each application as it grows in size will require endless tweaking unless you design it correctly to grow along with you.
For starters, a large system of say 2000 gallons and more will require about 3 watts of power per gallon where smaller ones may require 5 or 6 watts per.
It is important to know that the whole volume of a system is circulating so that areas of extreme temps do not form. and that wild swings do not stress your fish or damage your beneficial bacterial processes.
I have found that self regulating heat trace cable fits my system best.
Most Aquariums are used indoors, and most people keep tropical/ornamental fish in their aquariums. So most aquarium heaters are designed to maintain temperatures in the 80s and 90s with an ambient temperature within 10 to 15 degrees or so.
subjecting an aquarium heater to sub freezing or very cold temperature would result in a an over worked heater that would likely never meet expectations and may actually melt out and break causing more harm.
For just warming a few gallons for catfish to survive, a bubbler and low temperature pond heater might serve you better.
It might be easier to visualize three 100 watt light bulbs in a break resistant tube heating 150 gallons while being submerged. Yes 3 100 watt light bulbs can get very hot but not very fast if submerged in icy water and with a built in thermostat that's designed to regulate a given duty cycle rapidly turning on and off like aquarium heaters do.
Hope this helps.