It's definitely possible they could be affecting the water. People usually do a "test" before they put them in, put a drop of vinegar on the rocks and if it starts fizzing, it will effect the pH level. I do know that Neon Tetras really prefer soft water, and that pH level is too high for them, but I'm not an expert on changing it. You might try posting something in another post.
As far as ferts, people often say that getting dry fertilizers are much cheaper in the long run. There's plenty of places that people list on here that have it premixed for you. The grass should carpet with what you have now, for sure. If you do decide to switch out the rocks, consider getting some a little larger, it will make the DHG even more petite and really make it look like mountains. What kind of lights do you have?
Bump: *another thread, sorry