If your plants appear to be taking a turn for the worst, ask yourself these questions.
If your plants suddenly start to appear as if they are about to give up the ghost, the first question you need to ask yourself is "What's changed?" - and think about these specific areas:
Installed a new grow light or fan? Have extreme ambient temperatures outdoors been playing a part? Cold nights? Hot days? Check your min / max thermometer / hygrometer for maximum and minimum temperature and humidity readings and reset regularly. Check out the articles on optimal temperatures and vapor pressure deficit to increase your understanding of how plants respond to key environmental factors.
Check your water - arguably the most important additive in your garden! Measure the conductivity of your source water. Hopefully it registers below EC 0.4 otherwise you should seriously consider investing in a water softener and reverse osmosis purifier.
Are you ensuring that your water is the right temperature before feeding it to your plants? Around 64 - 69 °F (18 - 19 °C) is desirable. Any colder will shock your plants' roots. Any warmer will depete the root zone of vital oxygen levels. Make sure you test with a nutrient thermometer, or failing that, it should feel 'tepid' to the touch - neither warm or cold.
Get your magnifying glass out and check leaves (especially the undersides) for the presence of anything foreign and crawling! Check out our Pests and Diseases section if you locate something that shouldn't be there!
If you're using a professionally blended nutrient, it's VERY UNLIKELY that there's something wrong with the nutrients you're using. It's far more likely that you mixed them too strong or too weak, or that you're using a hard water formulation in a soft water area, etc. Try mixing up a fresh batch.
The Glass Ceiling
Sometimes shortcomings in your ventilation do not show themselves until your plants have developed and their carbon dioxide requirements have increased substantially. If your plants grow well until they are larger and beginning to flower or fruit, and then they suddenly seem to slow and stagnate, this is a sure sign that CO2 levels in your garden are becoming depleted. Consider using supplemental CO2 or increasing your ventilation, especially if relative humidity levels seem a little high (over 60%). Be sure to check out our guide to calculating the correct fan size for your garden.