By some non toxic concrete sealer such as Thoro-Seal. Concrete will leach alkali which is why its raising the PH. What concrete mix did you use?
I have done little research on Acryl60 to make sure its non toxic but I have seen pond owners online use it through some research. I would do your own research on it, I will too but that is a bonding agent and also makes the create water proof so there is no leaching. You need to mix it up in the concrete.
What needs to be known about create is that you want to put it on as thick as possible. Water makes the crete weaker and less dense, less water will make it stronger and more dense. The more dense concrete is, the less leaching there will be. You can have zero leaching from concrete if you make it dense enough.
Use some portland cement type 2. Don't use type one as its not meant to be in constant contact with water. Get some Sand/Topping Mix. Thats what its called on the bag. Mix ratio about 2/3 Portland to 1/3 sand mix. (Give you an idea, 1 bag portland to 1/2 bag of sand mix) Mix with very little water, you want the concrete to look like cake batter, thicker than a paste. After it cures 24/48 hours, you can coat it with a sealer such as Thoro-Seal. Although at that mix there should be very little to no leaching occuring.
Ty
