Set formwork blocks on the foundation or in concrete

no 2 meters but 1.75 meters

But it would be a pity if, in the end, everything was for nothing because you made mistakes while building, it did cost a lot of money after all, and tearing everything down again is certainly time-consuming and expensive. I would inform myself well beforehand and only then get started. Are you doing all the piping yourself as well? I would also plan a floor drain; that way you can mix and filter the water well and also let the water flow under the nozzles easily in winter.

Best regards
Sabine

I don't quite understand why you want to pour the floor only afterwards.

In the end, it doesn't make any difference in terms of work whether you first make a foundation for the wall and create the floor later or do both at the same time.

I would prefer the latter.

When we concreted our pond (about 10 m³), we poured the floor and walls (a bit higher than the max. water level will be) in one piece.

I informed myself further yesterday and also spoke with my civil engineer... Now a continuous base slab with appropriate reinforcement will be installed. Everything else is too uncertain for me regarding the fracture behavior of the slab. I would have to lay it floating, but if I ever get the idea of replacing the foil with another coating, I have a problem. I am now preparing the gravel bed and the formwork for a 20cm thick concrete slab.

Pouring it in one piece would be perfect, but for that I need massive formwork like in civil engineering, and that greatly exceeds the costs.