Will it still etch glass when very dilute? Will the reaction still proceed slowly? At what pH will the reaction stop completely?
From Wikipedia on hydrogen fluoride: "Although hydrofluoric acid is regarded as a weak acid, it is very corrosive, even attacking glass when hydrated."
On hydrofuoric acid: It etches glass "by reacting with silicon dioxide to form gaseous or water-soluble silicon fluorides. "
It seems fishtank glass can be etched even when the water is kept at a pH of 7.0. "You might try a razor scraper, to see if that will remove any of it. If not, there is a very high chance they are due to etching. Etching is caused by the action of water, especially acidic water, over a long period of time. "