The leaves start yellowing from the bottom upwards when the plant/s need nutrients, with you only having a small container the nutrients would have been used up pretty quickly. The plastic bottle you put over the top isn't doing the plant any favours; it will create an extremely humid atmosphere which will cause many problems such as rot and mould etc. The only time you need to provide a slight humid atmosphere is when cuttings are being rooted or seeds germinated. Like it says in the replies above, you should look into buying a small sodium lamp or some fuorescents.