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Since the leaves don't look curled at all and the veins don't look green in those yellow leaves I do believe you have yourself a nitrogen deficiency. Also the fact that its happening to the older leaves points that out. Did the other ones that yellowed fall off by them selves or did you cut them off?
If you were watering only for a long enough period of time, all of the nutrients way have been depleted. Not a big deal though. The yellowing just looks like the begening of a N deficiency. Just feed once with normal amount of nutrients and then water twice. And in case you didnt know, thats a generic feeding schedule. Feed, water, water, feed. Only water when soil is almost dry. Keep that up for a week or two and the color should come right back.
Are you adjusting the ph of your water and nutrients, also very important. Hope this helps, if not add more info to the post. Temps, humidity, medium, ph, feeding schedule. And if you can, I would get some better nutrients. Regular miracle grow is low quality with excessive amounts of iron that can cause problems for some.
Yes, keeping the ph in check is critical. They sell cheap to expensive ph testers, and chemical ph up and down. There are also many home products to adjust ph like lemon juice. Just search and find what works best for you. The better your budget is, the esier it is though. Nutrients are also vast in choices. Personally i have used the Technaflora Recipe For Success Starter Kit with great results, and it has everthing you need for around $30. Currently Im using General Hydroponics Flora series. Forgot to say, Ideal ph for soil is 6.5-7.0 and the Recipe for Success starter kit comes with one of each of the bottles pictured on the right and a small jar of cloning get you cant see in the pic.