I am reading Sunken Gardens by Karen Randall and this is what she says, "You can even use it to clean the water line on a rimless tank by drawing the water level down a few inches, applying the vinegar to a paper towel, and then scraping. Avoid getting large quantities of vinegar into the tank, but a few drops will not hurt."I do a quick wipe around the rim with a kitchen towel dampened with vinegar every WC. Works a treat. Any drips of vinegar into the tank water are insignificant compared to the volume of tank water, and not toxic at that level of dilution.