DOC's are Dissolved Organic Carbon compounds. They are produced as by-products of life. Because your tank is a closed system, they accumulate in your water to unhealthy levels, making the water "thick" and unhealthy for fish. There is currently no easy way to test for these, so nitrates are used as a proxy. In other words, if nitrates are high, you can bet DOC's are too.
There are also other waste products that are harmful to fish that you can't test for, such as pheromones. It is believed that pheromone/hormone accumulation may be responsible for fish "growing to the size of their tank." Their own toxic wastes are stunting their growth
I agree with DeRo, I try to keep nitrates below 10 ppm. I'll sometimes do two or three 50% water changes on my heavily stocked tank, but always 50% water changes every week on even my lightly stocked ones. There is some debate as to what constitutes "acceptable" nitrate levels. Some people say 12.5 is high, others say anything above 20 is bad. The point is, try to keep them as low as possible.
Do you have a liquid (drop) test kit? If not, you should definitely look into getting one. You can get a master kit with ammonia, nitrite, nitrate, and pH for about $15-30. GH/KH is a separate one that usually runs $10 or less. Don't use the strip/dip tests as they are notoriously inaccurate and a pain to read ("Is that pink or pink/orange?")
Good luck, Jason!