pH Spike, something in my tank?

[MadameDesu]

As some may already know, I got my API Master Test Kit in the mail yesterday and I've been testing all of the water I can find because it's kind of fun.
Anyways, I've been testing my tap water and tank water and found something interesting ...

Water fresh from the tap usually has a pH of about 7.2, but after testing the tank water today, it's was up to about 8.0! Keep in mind that I did a 100% water change yesterday, so it's got to be something in the tank!

Right now, the contents of the tank are:
Whisper 3i Filter
Cascade 7-watt heater
Pineapple House (made of resin)
A silk/plastic plant
A cheapo Walmart plastic plant

Any idea what it could be?