Plant Food/ Iorn Level Test Kits

[Fishyworld]

Hi,
 
In my tank I have x1 Java fern, x2 Chainsword, x1 Pennywart, x1 Green rush. I brought them all a little over a week ago, and in the last couple of days the leaves on my chainswords started to yellow and die, following suit is my green rush. The java fern and pennywart are so far fine.
I thought it was possibly a lack of iron, so I have put in 5ml of liquid plant fertilizer (as per instructions on the bottle) that contains 26% iorn and other trace nutrients. I'm hoping this will fix the problem as I don't really want to have to play with CO2 injectors as I am new to plants and am not sure how to properly care for them just yet. 
 
How often do I need to keep adding the fertilizer? The bottle did not say how often it should be added, it did say something about some sort of iron level test kit.
Do I need a test kit for iron levels and if so what is it and how does it work?
 
Also I added the fertilizer to the aquarium water, does it need to be injected into the substrate?
 
My substrate is a mix of sand and gravel, in case that makes a difference

 

[coolie]

The bottle should tell you how many ml per litre in your aquarium and how often to dose. It's impossible to say without the information on the bottle.
 
You don't put liquid ferts into the gravel. There are fert tabs for certain types of plants, but most aquatic plants feed through the leaves. You couldn't inject liquid into the gravel anyway, as the liquid is designed to work diluted to the correct dosage in the water and you would be attempting to create overly strong pockets of liquid, it just wouldn't work anyway.

 

[Fishyworld]

The bottle should tell you how many ml per litre in your aquarium and how often to dose. It's impossible to say without the information on the bottle.
 
You don't put liquid ferts into the gravel. There are fert tabs for certain types of plants, but most aquatic plants feed through the leaves. You couldn't inject liquid into the gravel anyway, as the liquid is designed to work diluted to the correct dosage in the water and you would be attempting to create overly strong pockets of liquid, it just wouldn't work anyway.

 
The bottle told me the initial dosage but not how much to follow with. However i've just been adding it in around every week and everything is going fine.