Iron does help plant growth, as does any other nutrient plants need to grow properly.
It's not the most important nutrient however.
For the sake of trying to explain quickly, I will classify the nutrients we give plants in our aquariums into 3 different categories: NPK fertilisation, traces (this includes iron) and carbon.
NPK fertilisation consists of the "macro" nutrients, the ones plants generally need the most of. "N" is for nitrogen, usually supplied in the form of nitrates or ammonia, from fish waste or using fertilisers. "P" is for phosphorous, which is either dosed from fertilisers or is supplied in fish waste (fish food contains it). "K" is for potassium, and this is either already in the water, or supplied using a fertiliser.
Next are the trace elements that plants need (in smaller quantities that the above nutrients), these can come from fish waste (from the food) and can be found in the water already too, and you can add them to the tank using fertilisers (most fertilisers mainly consist of mainly traces).
Next up is carbon, which is already in the water from fish, bacteria and all other organisms respiring, but more can be added from the addition of CO2, or liquid carbon sources such as glutaraldehyde (as used in Flourish Excel, EasyCarbo and the Aqua Essentials brand). Glutaraldehyde is well recommended because it benefits plants and seems to inhibit algae.
The point of me saying all this is, if you increase the iron in the tank, it will be of no benefit if a more important nutrient is limited anyway.
So your best bet for fertilising your plants without using CO2 is to add a liquid carbon source as well as some trace elements (use the lowest dosages for both of these) and then rely on the fish to supply N and P. If you use a fertiliser like Seachem Flourish that already contains a small amount of K for traces, then you may not need to add that.
If you notice any deficiencies, then add fertiliser accordingly. This is a good quick guide to plant deficiencies (and an excellent website to buy fertilisers from):
[URL="http
/www.aquaessentials.co.uk/index.php?main_page=page&id=8&chapter=0"]http/www.aquaessentials.co.uk/index.php?...8&chapter=0[/URL].
One other thing I haven't mentioned is circulation. It's important to have good circulation in a planted tank to distribute nutrients to all areas (where plants can block some flow), and eliminate dead spots where algae can be triggered. If you want your plants to do well over algae, adding an extra water pump (often a cheap powerhead will do) usually goes a long way, especially with extra fertilisation. Try to aim for a total of 10x tank volume per hour turnover rate when you add the turnover of all your pumps together, but don't worry too much about numbers, as most people get away with much less
.
Supplying just one nutrient that a plant needs (like iron) in excess will not help. It would be like trying to give someone more vitamin C and depriving them of all other vitamins.
I hope that answered your question
, if you can be bothered reading it all
.
If you want more specific advice relevant to your tank, you will need to post tank size, lighting wattage and type, filtrartion, substrate, water stats, fish and plant species
.
