If you're like me then you sometimes forget to empty the test tubes right after testing your water. I have found that the ammonia test will stain the inside of the tube a light yellow color if left overnight. This can make reading the results of future tests harder.
I tried everything from running and soaking in hot water, to scrubbing them down with the filter intake brushes. Nothing worked.
Then I remembered something I had learned to clean the stains from glass coffee pots. For the coffee pots you would fill them with ice then soda water. The Ice acts as an abrasive agent and the soda water moves the ice around against the stain.
Well that wouldnt work for the test tubes because the opening is so small so I thought about using salt instead. If you fill the tube to the top of the stain and then add water, cap it and shake for about 30 seconds to a minute, dump the contents and rinse. Most stains will be completely gone with one or two of these treatments. Its cheap and easy. You can use any salt that you would use in the aquarium as long as it is a larger grain such as aquaruim or epsom salts are. No more stained test tubes to annoy you.