This may be completely useless information as i grow hydro, but, last year i hhad 2 of 6 plants go incredibly yellow. I had no idea why, and nor did anyone on here!
It turned out my airstones were blocked, and my roots were getting no oxygen, therefore they rotted and died.
Basically what im suggesting is that you may well be overwatering to the point of rotting your roots??
The smell you mention, if it really is quite a nasty smell, could be the same smell i experienced from the rotting roots?
For you i hope its not the case, but i feel you ought to try and find out?
fingers crossed for you
'EVERYTHING in life you can either do, or not do.
If you cant do it, you can learn to do it.
But you will never be as good as someone that could do it in the first place'