Hey Growmie,
this a classic case of nitrogen toxicity. You can tell from the dark green, shiny leaves. Simple fix though ....Reduce the amount of nitrogen that is being fed to the plants. If you are feeding extra nutrients, cut down that amount. If you are in the flowering/budding stage, make sure you’re using a formula that’s specifically meant for flowering, or else it could have too much nitrogen. Remember effected leaves likely won’t recover, but you should see the problem halt with no new leaves being affected.
Hopefully, this was helpful and remember ..."Happy Growing" and "Free your mind one puff at a time"