ahhh pictures! looks like it might be potassium(K) deficiency, which is possible since you were using baby bio untill recently. tomato food with seaweed extract should cure it. id be tempted to use 1 capful tomato food in 2L of water and pour it around the stem of the plant. it should be okay to feed it strongly at the moment. old leaves will not recover, but new ones should be healthier.
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