Micronized iron
 

I read somewhere about micronized iron that you can add to the filter. I am
having trouble keeping the iron up in my tank. Anyone have any ideas?

I use a product by Nutrafin called "Plant Gro". It's 0.15-0-0 (NPK) and about 0.26% chelated iron with a few trace elements thrown in. A 4fl oz (120ml) bottle is supposed to treat 235 US Gal (890 litres) so it's kinda in the expensive option if you dose at the recommended rate. I generally use it to "top up" the iron from time to time and that works well for me. I'm sure there are many other varieties on offer.

I use it as well but I am way more cautious.

I only drop one capfull on my 33Gal tank once every two weeks.

Last time I felt adventorous and put two caps of it at the same time,
one of my ghost shrimp died. :)