Hi guys.
Just to clarify some things:
mopani is one of the woods to change ph the least, compared to mangrove/oak etc.
Second of all, every tank has its own balance, so it's not the tap water changing the ph of the tank water, but rather the other way around. Even with a 100% waterchange it's almost impossible to change the ph, once established in the tank.
Not to go into detail, just to mention it, without breaking down carbonates, adding CO2 etc. over a longer period of time, there is no sustainable way to change the ph.
And I have to agree to the others who already mentioned that nearly all fish are fine with higher phs as long as they have time to adapt to it. Better a high ph than an ever-changing one!