fish pond fish dying

Question
i have outside fish pond, rectangular size 1m x 2m lined with bitumin and half metre deep (900 lts water).  Pumps water into two bio filters, excellent filtration and aeration.  range of fish (old goldfish from previous pond, plus koi from various local nurseries) but one dying every second week. no visible disease. water looks good. water lily doing well.  been successful with various other ponds but this one not.  lost over 10 fish now, very strange. please give advice. thank you ernie sloan cape town south africa. can send picture of pond via e mail erniesloan@mweb.co.za

Answer
I'm sorry about your fish.  Photos might help.  I will e-mail you with my e-mail.  Have you tested the water?  I'm not sure what tests kits may be available in South Africa.  If you were in the US, I would say to test the pH, hardness, ammonia, and nitrite at the least.  Perhaps your pH is too high or too low, the water is too hard, or there is ammonia in it.  Without tests, there's no way to know.  If you provide more information about the pond, maybe I can come up with some possibilities for why the fish have died.  What is the water temperature there right now?  I wish I had an answer for you but I don't yet have enough information.  I hope things improve.