Will it still etch glass when very dilute? Will the reaction still proceed slowly? At what pH will the reaction stop completely?
"Although hydrofluoric acid is regarded as a weak acid, it is very corrosive, even attacking glass when hydrated.[1]"
It etches glass "by reacting with silicon dioxide to form gaseous or water-soluble silicon fluorides. "[2]
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. "
(Have a look at the image.)[3]
[1]https://en.m.wikipedia.org/wiki/Hydrogen_fluoride
[2]https://en.wikipedia.org/wiki/Hydrofluoric_acid
[3]http://www.myaquariumclub.com/white-cloudy-thing-on-aquarium-glass-648359.html