Backyard Aquaponics
A place to discuss aquaponics

does your mate have 2 or 3 growbeds that act as heat sinks connected to his spa
Yes Stuart its a bit like that

No grow beds, but he's also heating to 40+ deg. It doesn't stay at 40 for long when he turns the air on. I'm also assuming that anyone wanting to seriously give this a go would have their FT and garden beds well insulated and in a shed or greenhouse to even out the extremes.

Some rough calcs assuming my mates spa system is maxed out at 40 deg (which it isn't) gives a maximum loss for his system to maintain a system temp of 24 degrees of 780W over the full 24hour period. This is ignoring the extra losses that would be in his system due to the greater differences in temps and the extra performance gained by running cooler water through the panel. There would also be heat input from any pump but we'll ignore that. If you considered the garden beds as 2m² and not protected from wind (heat transfer coefficient of 12W/m²C) at 12degC you'd get 288W loss from the surface of the bed at 24degrees. I've ignored latent heat of vaporisation as realistically the beds would be in a high humidity greenhouse which would also stop the wind and probably be warmer than 12deg for a fair proportion of the day. To avoid temp spikes (night to day) a secondary storage tank with a simple copper coil heat exchanger could be used to provide heating at night. Further panels could always be added to supply more heat as required.
Not quite as simple as most would like and you'd probably need a backup for when the sun stops working but theoretically possible. Realistically probably not.

It's just lucky I like trout!

I think you have a lot more reading to do
[simple copper coil ]
any copper in an aquaponics system will kill your fish
Unless its a heated glasshouse when its 0 deg in the night its 0 deg in the glasshouse
Theres been great advances in aqua over the last few years made by people experimenting
give it a go but listen to the people that have been there done that