Question
QUESTION: Hi.I have a concrete pond(2yrs old)5000gl,acid treated and bottom painted w/rubber paint.Test results are always perfect, ph is 8.5-8.7. External pump 5200g/h,biofilter 6200g/h, uv lamp 57vt. I have 2 problems that never go away:as temp goes up-lots of foam all the time esp.at night and water always has suspended algae.

ANSWER: Is the pH of your tap water also in the 8.5 to 8.7 range?  If not, then the concrete might still be leaching somewhere (perhaps it is not fully painted somewhere, etc.).  If the pH of the tap water is the same, then you probably can't get it any lower.  A pH of 7.5 to 8 is ideal.

Foam is usually caused by organics from leftover food, fish waste, dead plants, etc.  If the pond is clean with little leftovers, not overstocked, and animals aren't spawning like mad (fish, frogs, etc.), then foam isn't usually a problem.  For some foam, doing water changes and adding fresh activated carbon to a mesh bag in the filter usually takes care of it.  If it's still a problem, you might want to install a protein skimmer (foam fractionator).  They sell them for ponds but they're normally used in marine tanks.  Also, there are anti-foam chemicals you can add to the pond.  

Since you have a UV sterilizer, you shouldn't have suspended algae unless it's not either the proper size or not getting enough flow through.  Or, the UV lamp needs changing or its sleeve needs cleaning(try that first).  Otherwise, you can try adding more shade to the pond and some natural approaches (plants if you can add those (you can protect them from koi if you have koi), barley straw, good bacteria, etc.).

Good luck!

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QUESTION: Protein skimmer!What a great idea!!!Noone has ever suggested it before.Do you trust any particular brand that is suitable for a 5000gl pond?

Answer
I've never bought a pond protein skimmer or really looked into it.  I did a quick search and didn't find anything.  I know I've seen them somewhere.  A marine tank protein skimmer may be adaptable for ponds but I think someone makes them for ponds too.  I just can't find it right now.