Dark green, shiny leaves are a sure sign of nitrogen toxicity, the solution is to reduce the nitrogen your plant is getting. 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. If you are not feeding extra nutrients, you may have “hot” soil that has been giving your plants extra nutrients. In that case, flush your plants with filtered, pH’ed water to help clear out the extra nitrogen.