Type of gravel? Standard aquarium stock. Had it for years (about 3.5 and counting). Of course, I've never tested it before. Kid in a candy store syndrome. Couldn't resist.
I did add a few rocks when I planted the tank. They were sold as river rocks from the same LFS, but its possible one (or more) of them could have been responsible for leeching.
And yes, I top off with tap water. I've tested it too. The KH is considerably harder, but the GH is much lower (9 as opposed to 16). At this rate, I'm tempted to start mixing it in with R/O water. Could that be what's making the difference?
Since my Nitrates were getting low (from 20 to 5 in a week), I did a bit of a water change tonight. I figured that would pump up the nutrients to some degree until I can get some ferts (everyone is out of them locally, so off to Dr Fosters I go).
I did a 1/3 water change when I added the plants, and close to a 1/3 tonight.
I should note that after the water change tonight, the GH of my tank dropped from 16 degrees to 13. And there had been minimal evaporation in the meantime.