Ph in my tank is too high I believe.

[Xr3]

I have a 40 gallon tank with 3 angelfish and a pleco. As it stands right now everything in my tank such the nitrites and nitrates are normal. I had 4 angelfish but one died this morning with what seemed no cause. I caught the death within 10-15 minutes of it happening and he was completely fine and had eaten about an hour or two before. He was not sticking towards the top of the tank or the bottom but swimming with the rest of the angelfish and then died within 10-15 minutes after I was watching them all swim around. I checked all the levels of the tank and the only thing that seemed to be off was the pH.. It usually sits at around 7.5 but today it is at about 8. Is the change in pH enough to kill the angelfish and whats the best way to lower it? the temperature of the water sits at 79 degrees 24/7. I did a water change about 2 weeks ago.. If anyone knows how to 1. Lower my pH down safely and 2. why the angelfish died. Please let me know! (they're Black angelfish)

 

[rlederer]

There are several reason why your angelfish might have died and pH might be a cause. See Troubleshooting pH Changes in Your Fish Tank - The First Tank Guide - How Can I Figure Out Why My Aquarium pH Is Changing? for a good explanation of why pH might change. A quick change in pH might be harmful to fish but why one fish died and not the others is a mystery and it might not be the pH.
That said, check your water source for the pH level and then check the tank regularly. If the pH increases, something in the tank - filter, gravel, decorations, is causing it.
Best way to get pH back to where it was is to change the water more frequently - at least weekly. Don't add pH lowering chemicals - that won't solve the problem.
Then again, it might not be the pH that killed the fish.