When noticing burnt spots on the leaves or burned edges (green beans, lettuces etc.), you need to check the electroconductivity of the nutrients enriched water located in the reservoir of your tower (this can be done with an electrical conductivity meter – every seriously committed aeroponic gardener should own an electrical conductivity meter).
You want to keep the electroconductivity around 1.8. since when EC gets around 3 and higher, this is when burnt spots and burnt looking edges on the leaves start appearing.
When the electroconductivity is too high (2.5/3 and higher), it means that the water has too much dissolved solids in it – as your plants take the nutrients out of the water they leave other nutrients behind… when too many dissolved solids (nutrients left behind by the plant) accumulate in the reservoir, plants cannot uptake new nutrients properly. Water must then be dumped from the reservoir to bring the EC down and be replaced with fresh water and new nutrients.