Since lettuce is made up of at least 80% water you can imagine that the water that they are grown in while in an AeroGarden can be very important. The most important thing is the temperature. When growing Romaine, make very sure the water is not too warm. Anything above 74 degrees and you will start having issues. If you see the leaves start to wilt a bit or droop that is most likely a sign that your water temperature is too warm. One idea you may try if you are having this problem is to top the water off with cold water instead of the usual room temperature water. This should be enough to make them happier.