Greetings,

My concrete pond (roughly 12ft x 10ft x 4ft excluding filter dimensions) was built, soaked for over 2 weeks, drained, then water jetted to remove loose sand and dirt, then refilled and has been running for over a week now (fishless). Here are my source water parameters: PH 7 and (rather high) Nitrates at 20ppm. Ammonia and Nitrites are 0ppm from the tap. Once it goes into the pond, PH goes up to 9+? (Max reading of tester is 9), ammonia and nitrite is 0ppm but I haven't got around to reading Nitrates. I am getting green water (I do not have UV).

So I have put some water lettuce and duck weed to hopefully soak up Phosphates and Nitrates. I also put in some mature bio media hopefully to seed the new biorings and japan mat. The green water does not bother me but the high PH does. I have yet to put my oyster shells in and am concerned as it might cause the PH to go even higher. Am I correct to assume the cement is still leaching alkalinity into the water and I just need to keep flushing the pond a few times and that should do the trick? What should I do? Put the oyster shells in to stabilise/ buffer the water at the expense of raising PH?