I now know myself why the plant went yellow, and why one other plant went yellow.
Basically they were dying!!
So now i have harvested the two that were dying. The airstones in the buckets had become clogged, therefore no oxygen was reaching the roots, therefore the roots die! Jeez, what a f**king smell!!
Still, now i have more room in my tent i am able to keep a better eye on the remaining 4 plants.

So a warning to others:

Unlike me, make sure you can get to the roots to check on them now and again.That way your plants should finish successfully.

Next grow of mine will only be 4 plants because 6 has left me so little room in the tent. I actually believe i will be able to match the yield of the 6 with only 4 and some training..

There you go, lesson learnt by me!

By the way, i quick dried a nice bud by wedging it in the heater vent inside my car for 20mins.
What a lovely smoke!! (even though from a dying plant)
Cant wait for the buds im drying out properly.........