I double dose my tanks with java ferns and they're still doing great. I think the best way to do this is just dose by what it says on the bottle, then slowly increase it daily so the plants will be slowly accustomed to it. By the way metricide and other glutaraldehyde based products are just carbon substitutes, not soluble co2. If you're going to dose with SeaChem's direction, just remember that there are different strength levels for metricide/cidex compared to Excel.
Currently out of the hobby
Forum lurking every now and then