Looks like either a nitrogen or iron deficiency.

I would suggest cultural problems rather than deficiencies. The plants are, I assume, indoors where basil does not get optimum light and air movement. The apparent fungal problem (browning lower leaves) look like symptoms of over watering and the leaf damage looks like spider mite which loves indoor conditions.

Here is a link that might be useful: Spider mite on basil

Excellent insight. I have had spider mite issues in the past though I haven't seen any on my basil plants so far.

I don't think I'm over watering though. I usually water when the leaves get a bit soft (so once a day). If anything I feel like I'm under watering since the soil drains very easily.

Once a day is almost certainly far too often unless your home is kept incredibly hot with no humidity. Stick your finger into the soil and if it feels slightly moist an inch down don't water. Leaves droop from overwatering as well as underwatering so check the soil, not just the appearance of the plant. It's easier to revive a dry plant than save a drowned one.

If you do not have grow lights you are pushing the limits trying to grow basil indoors in winter. It is likely to be sickly and sad.