Backyard Aquaponics
A place to discuss aquaponics

Ok, so for me an in ground FT will be just fine , also GBs on the ground...
As I am looking to a max water temp of 20c.
Around here the soil temp is about 14c(1200mm deep), as we have an abundance of trees, and a prevailing breeze off the ocean the temp at 6pm after a 35c day is about 18c droping to 10 - 12c over night, so we only have a hot day window of 11 hours thermal cycling (cool - hot - cool).
Yes temp gain over the warmer period will happen.

If you design a system ( ha I sure as hell did not), so that it has a lot more water volume and pumping capacity than you require for the amount of fish you want to stock, this enables you to pretty much stop pumping water during the hottest part of the day. Gives you a lot more control of temps.

+1 for burying tanks for temp stability. Who would have thought trout in Kal in october
The unburied front system is almost at trout death temps.

have you looked into water walls? If you have insulation on the North side and add water wall bags there, you will get extra thermal mass that is heated by the sun and air in the greenhouse during the day.
https://www.growerssupply.com/farm/supplies/ProductDisplay?catalogId=10551&storeId=10001&langId=-1&division=GrowersSupply&productId=109786

I do not think you want to couple your tanks or greenhouse to the ground in your climate, at least not for tilapia growing.

Now if you were to dig down a deep foundation and insulate around it and down and under the greenhouse and then re-fill with dirt, sand, rock watever, then you would have a huge thermal mass that is protected from the external ground which would help but probably not what you planned to do.

As for helping add thermal mass into greenhouses. I've seen many people put barrels, buckets, whatever containers filled with water to take up any extra space in the greenhouse. Often painting them black or dark to absorb more heat to give off overnight.

Definitely get some solar water heating going with heat exchangers or there will be little chance of keeping anywhere close to tilapia temps going on without huge electric bills.