why do you want to bring it down? are the fish all happy and healthy, if yes then just leave it be IMO.
If you do want to bring it down then the only really safe way to do it is to take stuff out of the water, not put stuff in. The reason the pH is so high is cos of all the salts and carbonates etc in the water. adding more stuff in is only going to cause more issuse. Seriously, unless you are going to filter the water and remove some of the bases then dont much around with it.
You can buy water filters these days for very little. Even a 2 stage filter will help remove some of the stuff in the water. My tap water comes out the tap at 8 but goes up to about 8.3 once the CO2 come out. Using a 2 stage filter it comes down to about 7.5, using a 4 stage RO unit it comes down to 6.2
pH modifing chemicals are nothing but a sure-fire way of stressing out your fish and will bend your mind when it comes to crazy swings they cause!
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