Backyard Aquaponics
A place to discuss aquaponics

If you have no other choice, irrigation hose can work.

I'll take it under consideration. But this will be my last resort due to thermal efficiency reasons. You might not know this but we in the Philippines have one of the most expensive electricity in the world. *sigh*

I am an experienced corrosion engineer. No paint can provide 100% protection of a metal in contact with water. Even if the coating was excellent, it will deteriorate in time and the fish will die. Excellent coatings will reduce the heat transfer as mentioned. The best heat transfer will be between a gold or platinum plate with channels for the fluid. Of course that is way too expensive. Speaking of expensive "Diamond has the highest thermal conductivity of any known material at temperatures above ~ 100K. The purest natural diamond single crystals reported so far have a conductivity of 24–25 Wcm-1K-1 at 300K, compared to 4 for Copper and 1.5 for Silicon."

I'm not a fan of any plastics because of pollution. Stainless steel is a good option. https://www.silvertubes.co.in/ss-stainless-steel-pipe-tube-tubing-suppliers-in-philippines.html

Sounds like a great place to setup some solar energy capture equipment. Consider building a Sterling engine powered cooler. They are far more efficient than photovoltaics.

I remember watching the ants walking in the shade of the curb in the Philippines. The ants were smarter than I was. LOL