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.