I had a good first crop of red and green lettuce but the second is showing dry discoloured leaves. Most leaves are inedible. (See photo) Temperature is 20 degrees and humidity no different to the first crop. Light cycle is 16/8 as it was before.
Other plants in the grower are also showing white discolouration eg chives and coriander. The previous coriander grew well.
Gathered from all the information and the picture it looks like you are dealing with a plant bug thrips. They can be hard to notice at first glance but as they are common houseplant and plant pests they will find a way into our homes eventually. These pests can come with fruits and vegetables from the grocery store or with any new houseplant. Thrips damage here is visible from the discolouration, pests are eating the surface cells of the leaf until the plant cells collapse, leaving small black spots there (droppings). I’d advise you not to consume the leaves anymore and reset your garden. Please find the instructions in the thread about thrips.