I have a concrete tank that I treated with a pore sealing chemical that is sold for Radon gas blocking. The one I used is 'Radon Loc' and 'Radon Formula #7' is another brand. This was a big mistake. These sealers have a pH of 11.3 and they will leach alkali for a very long time. In the 18 months I was raising fish in my tank, I could not get the pH below 8 for more than 24 hours. I tried phosphoric acid and acetic acid(vinegar). I could get the pH to 7.0 and 24 hours later it was back to 8.2. I tried several times. The bacteria were doing well at converting ammonia and nitrite but they could not lower the pH either.
Learn from my mistake and avoid using these sealers on your tank.
Dave Donnelly told me about Sani-Tred Permaflex which is a urathane. This looks excellant, but must be applied to unsealed concrete. It works by penetrating the concrete, so it needs porosity. My 24' x 6' x 6' tank would require 5 gallons which would cost $500 plus shipping. I could only do this if I sand blast off the concrete layer containing the sealer (I do not know how deep that would be). I wish I could go this route, but the total cost of sand blasting and the product is beyond my means.
I had a Radon problem in my basement (reading of 80 where more than 4 is bad). I used the Radon sealer there and it did a fine job (reading is now 3-4). I had some left over and figured it would seal the fish tank against water even better that a gas. It is food safe and it did seal the tank for water. Too bad the pH is so bad.
anyway, I thought I would post this and save others from this problem.
Thanks for the info.
You guys do the strangest tests.