How do you keep my fish tank from having such dramatic PH changes?

I am new to owning a fish tank, less than 6 months, and have 2 Goldfish in a 20 gallon tank. The problem is my tank PH has some pretty serious swings. I've seen numbers as high as 8 and as low as 5 in the last 2 weeks. Additionally I had problems with brown algae. I clean the tank about once every 2 weeks and the water temp is usually between 68 and 74, but I've seen it below and above these numbers. The fish seem healthy and active, but I am worried they will get sick or die if I don't address the issue now. Thanks in advance for the answers, if you need more info ask.