It sounds like leaf miners. It is not an ant but a type of caterpillar. It needs to be sprayed with BT Spinosad or something similar. Here is a link that might be useful: Discussion on Leaf Miners

You just very good advice from party-marty. Spinosad is the only thing I have found to stop leafminers. Home Depot in my area sell Captain Jack's Dead Bug, it's the only product that I know of at the big box stores that contains it.

Yes, the pot looks small and the plant looks underfed. You are giving it only about a quarter of its nutrient needs: Citrus Focus instructions say "Add 10ml of Citrus Focus in every litre of clean water, weekly. If leaves are pale or yellow, increase frequency of application."

Thanks Kelley! I'm still on the fence about how well my tree is doing but so far it doesn't look like there's any large infestations or obvious diseases. The only explanations would have to be root rot, lots of fungus gnat larvae, or not enough food for the tree. And I'm hoping it's not the root rot! With your mix, it might be due to the fact that each of those materials hold water very well. From all the research I've looked at online, vermiculite and perlite are usually recommended to be chosen for just one or the other in most soil mixes. Or a mix of both to equal 1 part. The only time I've seen a mix include both as separate parts was IVOrganics on YouTube but I didn't stay updated on how the tree fared. If your vermiculite was a larger, coarser type then it might be ok? I've read vermiculite tends to hold water very well due to absorption while perlite just gets wet on its surface. Is your lime also on that same mix? It probably is just the mix's drainage...I think an expert should give you a more informed opinion on your eureka. I've had my first lemon for only a month now. My advice would probably be to not listen to my advice lol