one of the best investments is something to measure the PPM. This way you know what your plants are doing. I have a 40 leter res and find that all of a sudden one day the water level will drop. When I measure the PPM they are very high 1800PMM+ and I have to add water to get it back down. The plants will go through cycles and somedays drink the water and other days the food. If the PPM gets to high it becomes toxic to the plant and will stress out the plant. I also flush for 24 hr each week when changing the res. It is also a good idea expecaily when the plants get better to check both PH and PPM about 3 times a day to keep every thing stable.
More stable the less stress the better the end results.