I once read about heat-retaining greenhouse design where the floor was two-level and excavated underground 12" to 18" if I recall.
The bottom level was like a wading pool but not very deep. It was intended to hold only enough water to be a thermal mass for absorbing heat during the day, and releasing it at night. There were pipes and a circulating pump, going through radiators or black 55 gallon drums.
The bottom level was made like the basement foundation of a house: poured waterproof concrete. The walls around the floor were 9-12" deep if I recall. I think the foundation walls were also poured concrete. Being below-ground probably helped give them strength against the weight of the water. Even so, I question whether cinder-block construction would be strong enough or water-tight enough.
Pretty shallow for a cistern!
There were also concrete footings of some kind to support the floor panels.
Then some kind of flooring was laid on top of the foundation and footings - poured concrete slabs? Wood or something else painted to resist humidity? The authors wanted a pretty tight seal so that warm water wouldn't fill the greenhouse with condensing fog all night.
The water kept the floor warm all night. During extra-cold snaps, the pump would circulate warm water through the radiators or the 55 gallon drums along the North wall.
In Texas, with a lot of shade, maybe the water could keep some plants cooler during the summer?
The New Alchemy Institute put very large fish tanks into small greenhouses, and grew tilapia.
