Straight coco no perlite? Looks like they want calcium in a major way to me. And you can check the light intensity with a free app on your phone. Keep the seedlings under 10k lux. 5-7k is perfect. Hopefully one of the more experienced farmers can chime in too.

Thanks for the reply - They’re in straight coco with canna coco feed. I’ve wondered about that, but keep seeing online that calcium deficiency is supposed to present as brown spots. Although I’m still curious if you might be right. I’ve gone over to RO water as the background ec of my tap water is really high - 0.9 sometimes. To get the calcium and magnesium levels right, people either use pure RO water and add cal mag until the background ec is 0.4 and then add nutrients as normal or add a portion of tap water, say 1/4 to 1/3, to get the background ec to 0.4, relying on the natural calcium and magnesium in the tap water.

I’m wondering if that’s causing the problem. Maybe there’s not enough calcium in my tap water to make up the right proportions. As you say, it would be good to hear what a highly experienced grower has to say..