Change nutes + Stretch question

Hello...

Im currently at my first grow.. 23 days into 12/12 ..



I have 4 plants.. Why is it that the one on the back left is stretching much more than the other ones?

Is there a rule of thumb for how long it will stretch?


I also have another question.. Im currently using Cellmax nutes but i wanna change to GHE Flora.. Do i just need to flush the system and then add GHE to the same EC as the Cellmax was?

Could be the one at the back is a main stem, main stem always stretches the most.