Thanks Ben for your great advice on the prevention of Mosquitoes, I found this product which has good reviews; Summit Mosquito Dunks.
Dale I have never heard of clarifying water with Moringa seed cake, I will research that further, thanks for your suggestion.
Elle, I have been considering my options with regards to sealing this project.
I am inspired by the work of David Butler in the UK. He left his media job to design and build organic pools - that is a pool with 50% plants and 50% swim area, no unnatural filtration and just a small air pump strategically used for circulation. There is not need for fish in this system, just plants.
Here are a few photos. A finished project by David Butler.
David's sketch for a pool suitable for a tropical climate
What you can see in my initial photo is the first stage of the construction, the dig. I decided to wait a season to see what happened to the naturally occurring water on my site. The water that has accumulated is either slowly draining away from the clay soil to meet the lower water table or this is the water table. Before I can proceed with my project I must be sure that this is not the water table. If it is and I decided to use a liner I would need to fit an extensive french drain system under the liner to avoid it being pushed by with the water pressure.
David uses liners on how projects, and if I were to do the same then the next step would be to build two retaining walls. One on the inner deeper hole and one for the bigger outer hole. This would then be covered with an insulating layer of something soft, then have the liner applied.
I have been toying with sealing this hole with cement (like the water tanks they build in India) or even using bentonite clay to avoid a liner altogether - liners are expensive and can be difficult to fit.
Well thanks for listening, thank for your comments and suggestions. I'll take all the help I can get.