I’ve no idea what the Kratky method is (well I do now because I’ve googled it) but I promise you these plants don’t look ‘very sick’. They have a couple of minor blemishes. If they were out in the garden you’d probably not even notice them. I’d eat the top one and just discard any leaves you don’t like the look of. I’m wondering why you chose this growing method which requires much more intense and precise input from the gardener than just planting in soil. I'm impressed. As a beginner you’ve chosen a more challenging method than necessary but you seem to have done very well so far.