Can hydrofluoric acid etch glass at a pH of 7.0? 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: &quot;Although hydrofluoric acid is regarded as a weak acid, it is very corrosive, even attacking glass when hydrated.&quot;
On hydrofuoric acid: It etches glass &quot;by reacting with silicon dioxide to form gaseous or water-soluble silicon fluorides. &quot;

It seems fishtank glass can be etched even when the water is kept at a pH of 7.0.
&quot;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. &quot;
In $\ce{SiO2}$ etching, there are two active fluorine-containing species, $\ce{HF}$ and $\ce{HF2-}$.

The rate of etching at 25 degrees C in Angstroms per second is:

$$9.66[\ce{HF2-}] + 2.50 [\ce{HF}] -0.14$$ 

according to A Study of the Dissolution of SiO2 in Acidic Fluoride Solutions J. Electrochem. Soc. vol. 118, pp. 1772-1775.

The relavent equilibrium constants at 25 degrees C and ionic strength of 1M are

$$\frac{[\ce{H+}][\ce{F-}]}{[\ce{HF}]} = 0.0013M $$

$$\frac{[\ce{HF}][\ce{F-}]}{[\ce{HF2-}]} = 0.104M $$

Because of the -0.14 term in the rate equation, only at enormously high F- concentration would there be etching at pH 7 (such as in this situation).  

This study was concerning high (~1M) concentrations of fluorine species, so it might not be completely relevant for low concentrations or very long time periods.