Greenhouse Seeds / Sensi Seeds indoor grow

12-2, the seedlings are doing fine, only the oldest leafs on the Holy Snow are turning yellow... maybe someone has an idea whats going wrong here? 16-2, The Chemdog is growing fast at the moment, the others are a little more slow, but they're all doing good. Also the Holy Snow of which the leafs turned yellow a bit is recovering nicely, with nice looking new green leafs.