You have three basic options – prefabricated (fibreglass or plastic), concrete or plastic liner. While concrete can be readily formed to any shape you want, it cannot be moved once in. The other two can. If you decide on a liner, I recommend paying the extra money for a proper pond liner rather than just the heavy duty black plastic. Something like butyl rubber or Xavan. Make extra certain there are no sharp bits in the surface of the pond base or wall. On hard surfaces a thin layer of sand is a good idea before putting down the liner. Some of the black plastic moulded ponds are excellent and with a layer of white sand or light coloured aquarium gravel or pebbles with a few stone slabs or river stones can really look a treat.
Apart from what the water you fill a pond with contains (scheme water will have dissolved chemicals), lizards pooping in it will add nutrients that encourage plant growth. If there are no plants in it, then air-borne algae spores will soon establish, turning the water green. This will happen quite rapidly if the water area receives direct sunlight. Alternatively, you can put plants in, such as water lilies, reeds and sedges, water milfoil and the aquatic Vallisneria usually called Vallis or Ribbon Grass/Weed. The latter grows rapidly, taking up available nutrients from the water. Periodic removal of a percentage of the Vallis will be required to keep it under control. This is a good thing because the biomass you remove contains the nutrient wastes of the lizards. This should serve to keep your water nice and clear although you will need to periodically vacuum out the insoluble waste matter. Using a garden hos as a siphon is the easiest way to do this but the outlet end needs to be lower than the bottom of the pond.
Mosquitoes can be an issue in outside ponds. Easily remedied with the addition of just a few small native fish e.g. gudgeons, gobies, galaxias, smelts, hardyheads, pygmy perch or the like. If they are small enough, the Mertens should show no interest. If you cannot get hold of native fish, then use the White Cloud Mountain Minnow. Do not use Gambusia as they will destroy any and all tadpoles that may hatch and try and develop in your pond.
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