Nitrogen Toxicity is commonly referred to as "The Claw" and is caused by over feeding your cannabis plant with nitrogen. Most people identify this issue by the tips of the leaves curling downwards like a claw. The foliage will also be really shiny and dark green in color. Nitrogen toxicity also slows down growth and eventually causes the leaves to yellow and die.
How to Cure:
As implied by the name, Nitrogen Toxicity is caused by providing too much nitrogen to the plant. During the vegetation stage cannabis plants need lots of nitrogen, making it difficult to over do. Once in flower however, they need much lower levels of the nutrient. If you see signs of the claw, check the label on the nutrients you are using and look for the N-P-K ratios. Either eliminate or lower the amounts of the nutrients with high N ratios in your feeding mixtures to reverse the effects. You will also want to flush your plants at this point, by only providing pH'd water for the next 4-5 waterings. Make sure you're using "Bloom Nutrients" once in flower to avoid this issue.