Is the tank covered? You can try to "seal" the tank as best as you can from receiving fresh air. Your "tank" naturally emits carbon dioxide, and if left in the water, it will pull down the ph.
Suran wrap the top and the sump.
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Alkalinity is the ability to neutralize acids. You have way to much and therefore you water is to much on the "base" side of the scale.
You need more acid. The vinegar that FLric mentioned is a good idea, but you have to add it slowly! It is a very concentrated acid.
It will counteract the "buffer".
I have never needed to add it, so I cannot tell you how much to try first.
If it were me, I'd try one single ounce and see if that makes a difference. Bring it down slowly, and critters you have in there will not benefit from a rapid pH drop, even if it is for their over all good.
And as you were already doing, the water changes. I'm thinking larger percentages wouldn't hurt.