Leaves Turning Yellow

Ok...so i started my grow about four weeks ago and the leaves have started to get a yellow tint to them. The pH has stayed at a constant 7.0. I have a feeling it is from over-fertilizing them. I accidentally put a lot of nutrients on them the first week. I will try to get pictures of them on here, but as for now I can only describe them. Some have burned tips but others are yellow mostly in the middle of the leaves. The Strange thing is that not all of the plants are affected and I started them all in the same container. Can anyone tell me what is going on?

Pics are worth a thousand words.

It doesnt matter if you started them in the same pot, like people, each plant has a different reaction to different problems, one plant could love being over nourished and cope with it whereas the next will tell you to get fu**ed and start dying.

Here are some pictures. I'm transplanting them into separate containers tomorrow. Also, does anyone know any good methods of getting rid of gnats? I heard that a layer of sand will kill them off...is this true?

Here are some pictures. I'm transplanting them into separate containers tomorrow. Also, does anyone know any good methods of getting rid of gnats? I heard that a layer of sand will kill them off...is this true?

here are pictures of the plants that are affected










what shall i do?