Spidermites will destroy your crop in days if you don't atack them viciously. probably, your weak plant has them and thats why its sickly - because mites are sucking its juices and laying eggs in the underside of the leaves.

I tried soap water, pyrexes, scrubbed the floor with chlorine, sprayed all the walls - there was always a few that survived and pretty soon the plants were full of them again.

I finally killed them off with Provado Plus, after they destroyed a good crop for me.