Yes, the concrete leaching will cause the pH to change dramatically. In smaller concrete inserts that were built inside of glass aquaria, I've seen the constructors fill the entire aquarium with water and let it sit for several weeks, dumping and changing the water every couple days. I'm sure there are some ways to accelerate the leaching process but I would think running it for a few weeks and then sealing it would do the job.
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Matt Campbell

Big animals, little animals, plants - right down to the sea itself. We need them, not just for their own sake, but because all this has to be here for everybody forever. Only one thing is certain: if we are to preserve our environment and save this priceless wildlife we need much, much more knowledge.
Harry Butler from 'In the Wild With Harry Butler' 1977

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