I'm not sure of what you're referring to when you say Elodea affects alkalinity when CO2 is low. I have witnessed biogenic decalcification in high light, low CO2 environments (i.e. my 55 g tank, haha) but the calcerous deposits are insoluble in the tank water and shouldn't really affect the water chemistry by much, if any.
At the light level you are at right now, you will have a somewhat difficult time growing most stem plants with the exception of the Elodea you already have. The reason your light buzzes is because it is a magnetic ballast running at 60Hz - what your AC outlet provides. If I could suggest you get a magnetic ballast (which runs at 20,000Hz and is far more efficient) I believe you'd be pleased. If you want to try your hand at overdriving the 15 watt light to 30 watts, read my post on overdriving Normal Output fluorescents in the lighting forum on this site.
Hard water seems to be a problem at first for planted tanks, but I've come to find that hard water can actually be easier for high tech planted tanks than soft water. The buffering capability of hard water prevents drastic pH shifts when you add CO2, and the minerals in the water are usually beneficial to plants. I keep many varieties of tetras in my hard tap water, which is about the same as yours. CO2 injection brings the pH down to 7.2 and the fish and plants seem very happy.
- Sam P -
plantedtanker in limbo - all tanks currently in storage