If you are only changing "a few inches" every week, most likely very bad water.
Really don't care about pH at this point, but it's very important that we find out what your nitrites, nitrates and ammonia readings are.
Until you can get it tested, you need to start doing 50% water changes -- half the tank water -- every day until you can get a test kit and find out what the water is really like.
I'm assuming that your measurements are for a 20g tank, so you need to change out at least 8g each time you change the water. The water changes need to be kept up in that quanitity and on a weekly basis as well. Goldfish are heavy waste producers, they need their water changed in large quantities and a lot.
With a goldfish in a tank that small, you really can't slack off on the water changes at all. If constant water changes are difficult for you, it would be better for the fish in the long run if you could move him to a 30g tank and either a filter rated for 60g, or two 30g filters.
Roan