go ahead and cut the whole leaf off. Sounds like either a potassium or one of several micro nutrient deficiencies. I would bet my money on potassium, as that's ALOT more common. Are you seeing pin holes that are yellow and/or yellowed/withered edges on the leaves? If so, then it is probably potassium.
SeaChem makes a good potassium fertilizer that you can pick up a big bottle for like $10. On a low tech tank, dosing the recommended amount after every water change will keep this from happening. The brand I use is a little harder to find, and is a few dollars more a bottle, but very excellent. Brightwell Aquatics is the brand and the potassium supplement is called Florin-K.
If the leaf is only slightly damaged, leave it and dose your fertilizers. Potassium is a mobile nutrient that the plant can "reclaim" the damaged tissue. Meaning if you add what it is missing, the plant will heal itself.