Cement in a pond will raise pH and kill the fish.
When I first started water gardening I went into pond chat boards to learn what to do and not do and cement being toxic was a universal truth. The idea was cement has a high pH, around 14, that it would raise the ph of the pond and kill the fish. Logical and repeated by everyone in the chat boards.
I sure kept cement out of my pond and sure enough my fish didn't die.
Then one day I had a new filter design I wanted to try but the test pond was a bit too deep. How could I raise it off the bottom? The only thing I had was cement blocks. Well, how long would it take for this cement block to rasise the pH and kill my fish? I thought it couldn't be instantly so I decided to chance it. I would test the water pH a few times a day and when it started getting too high I could remove the cement and add acid to lower the deadly pH.
An odd thing happened, the pH didn't change, at all. Then my brain's high gear kicked in. When did people start using rubber liners? 20, 50 years ago. What did they make small ponds out of before that? High gear brain said "why cement of course you idiot". For hundreds of years ponders have been happily using ponds made completely of cement and suddenly it's toxic?
For the first time I decided to think this through a little and also do some research.
How much cement power right out of the bag would you have to toss into a pond to raise its pH? A lot. But this isn't the case with cured cement. Only the surface is exposed. Yes cement is porous, but not exactly rivers of water running though it. A cement pond will hold water pretty well until it cracks.
And cement doesn't exactly dissolve in water either. Put a cement block into a pond and pull it out 50 years later and it's still basically the same cement block.
The only actual research I could find had to do with concrete sewer pipes. They tended to break down in the low pH sewage. But they were talking about pH in the 5 range, weak acids. At pH in the 7 and 8 they saw little damage. Most ponds are 8 or 9 pH, even 10.
Turns out cement tends to seal itself. Its surface does react with the surrounding water an kind of "rusts" which then some what protects the cement beneath. This is why cured concrete doesn't dissolve in water. Acid or low pH water will dissolve this "rust" protection. But this isn't the case in ponds.
I then started using fake rock many from cement in my ponds and mortaring rocks to the bottom. No problems with pH.
One other myth was to "age" new cured concrete with an acid wash before adding water. Logical. I did that treatment in several ponds. But now I was sceptical and tried a pond of soild concret over liner with no acid wash, not even rinsed out. No pH difference with liner only ponds. No pH issue at all. And this makes sense. The "aging" with acid only removed the protecting rust layer. Acid had the oppose effect, it renewed the cement surface. Aged look sure. But chemically an acid washed surface has more potential to raise pH than unwashed. But even that didn't change pH. It simply isn't enough material to raise pH.
This experiment ended my believing all I read and heard about ponds. For several years I pounded away in the chat boards about my results. Slowly, very slowly, a few
people did come around. The cement myth still persisted but at least was no longer considered a universal truth. And this this was a very concept to test. But many people
would rather appear right than be right.