Koi ponds: Concrete pond or flexible pond liners, which is the best?
Hi Mary ... If you are building your own koi pond with your own bare hands and you are not experienced builders, then the flexible liner option is a lot more foolproof. I would use rubber (Firestone) rather than PVC.
If you do have building experience or you have skilled labour to hand then a pool built out of concrete blocks and then coated in two coats of fibre-reinforced render is a popular option. The render must be neutralised or sealed with water-soluble epoxy paint (this is my favourite, and its very robust) or an animal friendly bituminous product (purpose manufactured!). Before doing painting any sticking out fibres should be burnt off with a blow torch.
In the UK, for added strength in cold weather, and a smooth rock solid sealant around fittings there are teams of professional fibreglass technicians that do an amazing job. This finish has to be applied by people that do it constantly for a living, otherwise it can be an absolute disaster area.
Good luck and good digging!
Pete May