Great thread, great pics, love to see the progress! An in ground reef pond is a great idea that I dream about here in Southern Florida (number one American destination for European tourists).
Yes, the earth can buffer (smooth out/help ease) the summer high and winter low temperatures. Some aquarists here use water coils deep in the earth to provide geothermal cooling and temepaerature stability. However, I wonder much it can buffer the display tank when it seems so shallow.
Also Vitor, you put insulation around the tank, which isolates the tank from the earth’s temperature buffering. I can see the need for some cushioning material to allow for expansion and contraction of the tank and the surrounding earth â€"œ so that the tank does not crack, but too much insulation may reduce the buffering effect. Howe did you know how much insulation to put for safety, but not too much for insulation?
What is the display tank made of?
Why isn’t the glass top removable â€"œ what benefit do you gain? Why isn’t the top simply an inch wider so that it rests across the top of the reef pond?
Of course, with so much sunlight, did you consider biological filtering with an algae refugium?
Which brings up another question â€"œ are you feeding the Tangs algae, algae flakes, dried nori seaweed or fresh macro-algae from the ocean?
Have you thought about connecting the system to a small display tank in the house? There you could shelter and observe new species?