Ive only had that problem once a looong time ago. Went out and got some Technaflora thrivealive B1 micros and it cleared right up. Its just a light supplimental nute- 1-1-1 with .09% soluable iron. the vitamin B1 added will help give your roots a boost and protect them from disease too...just my
for some reason I remember stickin a rusty iron nail (not zinc galvanized) in a houseplant once and that kinda worked.