are you lining this pond with a liner,fiberglass, polyuerea, paint or other? altho pricey, if i could afford it..the polyurea would be my choice!
The finished coat is a product called heydi K11, it is a fish safe product that is painted thickly onto the render. the manufacturer recomends filling the pond, then let it sit for 9 days to a fortnight, then emptying and refilling, and check the ph, if the ph is not right, repeat the procedure. I have never used this stuff on a pond before, so I'll keep you updated on how it performs.