No, I wouldn't worry. Lava rocks are full of iron. Reddish rocks are usually red because of oxidized iron. There is even a type or iron found in limestone (limonite). Iron is a very common element in nature, and I wouldn't worry about it at all. You're not overdosing a liquid fertilizer concentration or anything in the tank. If you're that worried about it, don't do it, your tank will be fine either way. The TDS meter wouldn't tell you much, because iron contributes very little to the overall TDS of your water.