Pics wud help a lot. How old are your plants. wot temp do you have and what stength nutes did u use. Also which leaves are turning yellow. The very bottom leaves often go yellow after a few weeks. It happened on most of mine, if this is the case its not that much to worry about, although i'm not 100%
I have been a little lazy on posting the pics... This is day 13, and i think it was just a little nute burn on one, but its recovered nicely, just a little behind in size but right up there with the biggest in terms of maturity. I am giving half strength Schultz Expert Tomato Food. It is 9.10.15 . Their temp has been around 80-84 and i just put my ac back in the window so its hangin around 76 now. And now, here are the pics...
This is the plant with the yellowing, Plant 1
Plant 2
Plant 3 - The biggest so far
Plant 4 - The runt
Now, Some Group shots, look how the fan has made them all grow leaning =) Gonna be some thick stems!