Today I cleaned out my overflow. I removed my stand pipe to clean out the slots in the strainer. The strained i have on my durso stand pipe is about 12" long, it was caked with dirt and calcified stuff , like 1/8"-1/4" thick , it was really getting clogged , glad I cleaned it, maybe it was the cause of my nitrates, and maybe my tank would have overflowed after a while . I have never cleaned the overflow since I started the tank over 2 years ago, lots of yuck at the bottom too.