Rochelle and Gail:
What you need is a micronutrient fertilizer. It typically has no
N-P-K but does have iron and other metals. You only use a little.
Try a nursery place or if you are near a large urban center, try a
hydroponics supply place (or mail order). You want something
CHELATED to keep the iron from oxidizing right away. That and an
iron test kit and you should be in good shape. You may have an
inherited iron deficit, so you may need double doses for a while. I
was way behind, I just added enough iron to make 1ppm and overnight
it dropped to .2ppm. I used ferric sulfate (human) supplement
tablets (1.5 tabs in a 50 gal tank). The plants look redder already!
Dave Gomberg, Experimenta San Francisco CA USA gomberg at wcf_com