Why is my pH going up?

Question:

So why is my pH going up? It's been low for 3 weeks, and now out of nowhere it's changing.

Answers:

EDIT: Even though your test strips may be working now, I strongly suggest getting liquid test kits as they are far more accurate and last much longer. API is a good brand.

What fish do you plan on keeping? Some fish like a more alkaline pH.

Are you on well water? Well water is commonly higher due to the mineral content. It's also common to see nitrates in well water. As long as they're low, it's not a problem.