The cheapest (to run) and most efficient way to heat a large predatory pond is a water heat pump as these have up to 16X COP. So 1kW of electrical power will produce 16kW of heat! Your pond/tank needs to be insulated regardless of how you heat, especially the top which will lose 70% of heat. Heat pumps can be expensive to buy though. I only use heat pumps on bigger tanks & ponds (over 80,000L). With a long warranty and local service support & spares. I have 4 days before the thermal mass of my tank(s) start to drop.
I run clown loaches in above ground intex swim pools with a clear plastic cover. These are 10,000L volume and I use direct heat solar panels used for swim pools to heat these. Stay away from the flimsy strip pool heating, use robust heavy duty like OKU (German brand). I estimate one panel provides 1kW heat. Slow recirculation during daylight. Turn off water at night. You can get temperature controllers to speed up or slow down pums water flow but I prefer less automation.
https://www.ecoonline.com.au/performance-oku-solar-pool-heating-system-automatic-kit/
Some suppliers will not selll these for aquaculture or pond use so you may have to purchase without warranty. (Swim pools have chlorine). The OKU brand is fantastic. I can walk over, not worry about hail storms, UV degradation etc. I use 10 panels in series on the roof of a fish shed (300+ tanks).
Another option is solar evacuated tube collectors which would work but seemed overly complex. I have plenty of strong sun where I live so this option was overkill. Easily capable of boiling water.
