Hi all, I live in a coastal spot in New Zealand called Tauranga where the climate is pretty timid all year around. I had a little spare time on my hands this summer so I decided to create a water feature. With no experience and a heap of enthusiasim I did a little investigating and chatted with a few people about good and bad things to take into account. I did everything by hand including moving and positioning rocks ranging up to 200kgs into the furthurest point on my section with very limited access. It was a long drawn out and sometimes painfull battle but the perseverance paid off in the end. About 6 weeks later and I have a little under 2000gal Firestone lined pond, running a "Pondone" Clari Tec 10000UV filter with a 24/7 waterfall supplied by a 6000l/hr pump (I'm actually only getting 3600l/hr due to head pressure resistance). I used mortar to lock in the rocks surrounding the pond and concrete to fabricate parts of and lock in the larger rocks for the waterfall. I washed everything down well and pumped out before filling the pond. I then ran the pump for a week before introducing 5 Lilys and one Lotus. A week or so later I added a 2ft fish tunnel & 4 (50mm) Comet goldfish. They seemed happy enough after 4 days so I introduced 6 Fantails (120-160mm) and a few buckets of fresh water weed from a local river. I have since had 3 of of the comets and 4 of the fantails die over the span of a month. I purchased a testing kit and found the only thing out of spec was pH which measured 9 (or more) on the high pH scale. Everything else (Ammonia, Nitrites, & Nitrates) were all 0. At this stage I also realised that I had the Lotus sitting on a cinder block and the pump sitting on a large paving stone (to keep it off the bottom). Over the first month, I swapped out A LOT of water to try and drag the pH down as advised by a local store owner that I got my gear through. Tap water here is pH7.6 and the pH can vary a lot over the day (7.8 first thing in the morning and 8.4 late in the afternoon). I have resorted to isolating my waterfall out of the equation, lowering the level of the pond water below the rock boarder and recircing straight back into the pond from the filter. This appears to have worked for now (no deaths in 8 days) and my las 3 remaining fish seem happier and more active than ever. Eight days ago I swapped out approximately 2/3rds of the pond water and 14hrs later tested from both the water that was pooling in parts of the waterfall and the now isolated pond. The pHs were 9+ & 7.8 respectively at 8am. I concluded that the high pH had to be coming from the concrete. It has been disturbing seeing the fish die off after putting so much effort into making a haven for them. I thought this was going to beautify my yard and make a nice relaxing spot for me and the family to enjoy watching the fish playing in a tranquil setting. Instead it has been countless hours of stressing, changing out water, collecting dead fish, masses of phone calls, trolling the internet looking for possible causes/solutions and all the while wondering who was going to die next. The fantails were all quite large and given to me from a workmate. He has been good about it probably sensing how gutted and frustrated I am to lose the beautiful healthy fish he gave me. He is prepared to send a few more fish my way but I'm wanting to wait until the waterfall has been in use for a few months with no deaths. I have purchased a small container of potable concrete sealer that I will be using to apply a few coats on to the morter of the pond boader and every concrete surface that I can get to on the waterfall during my next days off. Any other advice would be greatly appreciated. Regards, Marcus.