I used to have a potted Meyer Lemon tree where problems would come and go as the seasons changed, Being potted presents several problems when it comes to a balanced feeding, watering schedule, and a reliable amount of direct sunshine, plus adequate re-potting when the plant gets too big or root bound. Below is a site with a wide variety of lemon tree pictures with leaf ailments which may be of help. Eventually I transplanted mine outside and it did much better.....before it was killed by a freeze :-(.

http://ipm.ucanr.edu/PMG/C107/m107bpleaftwigdis.html

A couple things: when you say grown indoors, is it indoors under a light or indoors at a large window? What are ambient temps where it sits? How wet is the medium remaining before you water it (do you water on a schedule or when dry)? Is this a self-watering pot or is it in a pot above a saucer and do you drain excess water?

It's indoors near a big window, facing the south side. The temperature at the window is about 70-80. I generally check everyday using the knuckle thing. I get it to dry out slight then deep water it. I don't have a saucer for it, unfortunately right now. & yes I drain the excess water.

The leaves resemble iron chlorosis but I'm just comparing leaves. I dont know how to check since I'm still a beginner.

When you say the knuckle thing, are you saying you're checking down to the second knuckle? How big is the pot versus the root ball? If it's a lot bigger than the root ball, then the surface can appear dry while the soil is staying wet lower down. That can cause problems and some leaf yellowing depending on the tree. Another thing to look at is a temp drop at night, does the temp stay between 70/80 or is there a drop? I've noticed that citrus indoors seem to like a nighttime temperature drop of about 10 degrees. Before I started adding fertilizers and chelated iron, etc. I might start here: 1. check lower down in your soil to see if the soil is staying wet. (If you have your inner pot in one of those drainage free decorative pots, this can be a real problem where the surface dries due to evaporation, but the bottom stays moist. Roots need airflow, and so does evaporation, so you want to make sure that they have that. If not, you can have wet roots. (Citrus also like some humidity, I've found the best way is to elevate the soil level above your drainage tray either internally or externally and then simply let the water drain off into the tray and evaporate on its own.) 2. Check the PH of the water you're using to water your tree. An easy way to do this is to take a glass and fill it with water. Add some vinegar. If it fizzes, your water is alkaline. If it doesn't, dump it out, rinse it out, refill it, then add a bit of baking soda. If it fizzes, your water is acidic. If neither react, your water is likely neutral or close to it. If you find that your water PH is alkaline it is likely causing some issues. Add a bit of vinegar to neutralize the alkalinity and water your tree with that. If that helps, then continue doing it and see if there is an improvement. 3. check your ambient temperature. Remember something about trees, they are not going to react immediately, it can take several weeks to see a difference when you make a change, so give the tree time when you fix something before you decide whether or not it has improved. Trees can also take a lot, and recover from a lot, so don't give up. Once you learn the needs of your own plant and your environment, it's not too hard. And don't stress over it. It is possible to "love your plant to death", correct when an issue presents, but then leave them alone and let them be. Others on here might be able to offer more advice, but that's my two cents.

Actually, I believe that HU's tree is nothing to worry about. It looks quite healthy. You seem to be doing a good job on this tree.

Thank you, everyone. I checked by the second knuckle, and its dried out like the rest but I just checked my ph of water and it was alkaline, so thats probably the problem. I just went out to buy a saucer for the tree, so it can get better humidity because the humidity in my area & house is low.

I appreciate all the help! Hopefully, the change in water and humidity help it to grow even stronger. Even though; it looks quite healthy right now!!

Oh. Okay. So should I just try some soil sulfer? To make sure my soil gets slightly acidic? Instead of trying to figure out completely.

I wouldn't worry about a PH meter or soil sulphur. A small amount (read about a tbsp per gallon of water) of vinegar added to the water you use to water the tree and you'll be fine if your water is alkaline. (Basic water doesn't mean basic soil, but some trees are a little sensitive to the alkalinity in the water. Vinegar just reacts with the basic elements and brings your water closer to neutral.) Often by worrying about it too much and doing too much to the tree, you wind up doing more harm than good. Your tree should be just fine. Your leaves are a little bit yellow, but that happens sometimes. Hence my comment about check if your soil is wet, check your tap water, check your ambient conditions and let it go. Trees survived in the wild long before we got involved and nature is a lot less fussy than we are. :)

So, I have been taking care of it with slight amount of vinegar. The two top leaves have corrected slightly and became slightly darker green in between the veins but remain lighter on the edges.

The problem now is that the top leaf that was growing in the past week, is extremely yellow besides the tip facing towards the main stem. Also, The brand new growth (2 itty-bitty leaves) is brown and looks dead.

Is it possible to get a picture of the full tree+pot? It is possible, if the rest of the tree is fine, that your problem may be transient and correct itself once the days start getting a bit longer.

Not exactly sure if these pictures are okay but I appreciate the help!

All in all your tree looks healthy. If you live more north and aren't using supplemental light, your tree can sometimes show nutrient deficiencies due to the short days. (My Nagami was really bad about this until I got a light for it). But they'll usually come back as soon as the days lengthen. If you're concerned about humidity during the colder days, just put your pot up on some pot feet inside the drainage tray. When you water, let the water drain into the tray and leave it. It won't look good, but as long as the top of the water doesn't touch the bottom of your pot, your tree won't re-absorb that water and the salts in it. That can raise the humidity level around the tree a bit as the water evaporates. Other than that, just focus on keeping the tree alive through winter. If correcting water PH with the vinegar is helping, keep it up, and then don't worry about it. Looks like you're doing ok.

Alright! Thank you for all the help! I'll keep at it and just push through til winter ends.

Glad to know it's not exactly a me problem as of right now.

Have a great day & Happy Holidays!

No problem. I should also mention that you want to make sure that as much of the tree and pot as can be is above the level of the window so it can "see" more light. That can also make a difference. Good luck and happy holidays.