The 10 gallon that I'm cycling needs more gravel, I didn't have enough at the time I started the cycle almost 4 weeks ago. When would it be safe to add more? I was hoping to add it at the end of the cycle when I do the 80% water change.
It shouldn't hurt anything to add more gravel now. Just rinse it off good and put it in there. It could skew the results or your tests for maybe a day or two but on the whole shouldn't affect your cycle much at all.