Agreeing with Kasa on this one. Although it may be ultimately impossible to know what actually killed your fish, it it highly suspect that the Ph fluctuations did him/her in.
If your tap water is 7.4 and you are trying to keep your tank at 7.0 you are doing more of a disservice to your tank by tinkering with chemicals. Whatever fish you have that like 7.0Ph will be just as happy with 7.4Ph.
You have a win win situation staring you in the face. First of all, stop using the Ph powder. From now on, treat your incoming tap water as you normally do but omit the Ph powder. Over the course of several water changes you will have safely stabalized your Ph to a constant 7.4 which will be much safer for your fish than trying to chemically alter everything. Not to mention you will no longer have to spend $$ on your powder.
I have WHite Clouds in my 20g and my tap water is at 7.6Ph and my fish thrive.
I will ditto what was quoted earlier....A stable Ph is better than trying to maintain a perfect Ph. Especially in your situation where your water is so close to "perfect" straight from the tap.