Question
Hi -  I know you have dealt with loads of green water issues previously, but I am now at my wits end having tried everything suggested.  My pond has a high ph level, but everything else is okay.  We also tested the tap water but the ph on that is fine.  We have a second smaller pond and the water is crystal clear!  Our main pond was built in spring 2009 and is a T shape.  Its 2000 gallon in size.  We have a pressurised filter with UV - bulb was changed in May and is working.  We also have a pre filter and a large OASE Aquamax pump.  There is a second pump providing a fountain.  We have 10 x12" koi in the pond, Lily coverage on half of the surface and a gazebo over the pond to provide shade for 2/3 day.  We have tried barley straw, chemicals, water change, salt - all to no avail!  What else can we do? - would adding another traditional filter, pump and uv help?.  How can we get this ph level down (its about 9).  Any advice would be much appreciated.  I think the fish are okay but we can't see them!

Answer
The cause of you high pH is most likely a high alkalinity, or KH.  It is usually caused by an influx in carbon, often from the debris on rocks.  It would take some hydrochloric acid and a large water change to fix that issue.  You need something to actually eat away at the alkalinity, not just (1+9)/2 = 5.  It's a lot more complicated than that.  You have to alter the buffer.

If you have two ponds on the same property that are experiencing different results, you are in a prime position do do some effective troubleshooting.  My guess is that the larger pond has less plants per volume, more fish per volume (or no fish at all), or gets hit by the sun for longer each day.

It sounds like you are doing everything right, by providing the shade and trying salt, etc.

Plants and algae thrive off of nitrogen (ammonia, nitrates) and other nutrients, or elements.  There are two more approaches you should try.

1) Strains of bacteria are responsible, in every pond, for turning ammonia into nitrate, then into nitrogen gas.  Plants also handle some of the load.  Where plants do not, algae does.  You can try to provide an excellent biological filter for these bacteria, so that the available ammonia is quickly converted and on its way out of the pond.

2)  Every organism needs energy to survive.  If you eliminate that source of energy, you eliminate the organism.  Nutrient control is a very tough proposition.  Almost every nutrient requires a process, such as ion exchange, to be removed from the water.  A good first step is to have your water tested by either the municipality  in which you live or by a private service.  It is more important to have the tap/well water tested, as that is what will be used in the pond in future refills.  You may as well get your pond water tested too.  It may reveal that some rocks or substrate you have used are leeching carbon into the water.

p.s.  Algae thrive in very narrow windows.  A temperature change of less than 5 degrees may get rid of it; or make it worse.  Same with every other parameter.  Getting the pH down may just be enough to make it disappear.