There's one on the left there that has a big black spot, Chris. It might be one of two things but neither of them are good news.
It could be sunburn - is your greenhouse in direct sunlight? The plants might need a little less light in the middle of the day? Has that spot been there a while?
OR it could be black rot, a leaf rotting fungus that can spread very quickly. If the spot developed, and gets bigger quickly CUT THE LEAF OFF well back of where the spot is, and get that leaf out of the greenhouse. Any leaf that has no green left on it could be cut off, it's not doing the plant any good. I would dust the cuts with a little bit of cinnamon to prevent fungal infection invading.
Have you tried adding a little bit of Epsom Salts to your soluble fert spray? I'm not sure but a couple of those leaves look like they have a little bit of a reddish tinge. This is a sign of magnesium deficiency. If you let that go too long, the leaves quit making chlorophyll (the green stuff that collects light and makes food for the plant), go yellow and die off.
If you are spraying them twice a day, make it always before noon so the plants have a chance to dry off before night time. You don't want the leaves to sit with water on them overnight.
