Don't focus too much on small details and be patient about the tank development. To get better advice please use the "I Need Help Template" that will give the other members better idea of your tank - see it here
. Here is my take on your tank:
I assume you maintain proper temperature and have good circulation so I'll talk about the nutrients. You have a lot of light so you really have to make sure your plants never run out of nutrients.
First make sure that your CO2 concentration is right. A good number as you probably know is 20-30 ppm. Use the pH/KH table for that calculation but verify your readings for KH and pH because often the probes and test kits give incorrect readings.
Same goes of N and P - make sure you have N= 5 to 10, and P= 0.25 to 1. Verify your test kits...
The iron concentration: Testing the iron in the tank water can be misleading and frustrating because once added to the tank the iron quickly turns into an unreadable form or gets used up. A good approach is to add iron every day as oposed to once or twice a week. How much to add every day? Perform this simple test:
- Take a one gallon jug of distilled water (from any grocery store)
- Add 1 ml. of Fluorish Iron to the jug and shake well
- Use your iron test kit to check how much iron you have in the jug as a result of adding 1 ml. of Fluorish Iron
- If needed add more Fluorish Iron untill you get a reading of 0.1 ppm.
At the end of that test you will know how much Fluorish Iron you have to add to 1 gal. of water to get the iron concentration to be 0.1 ppm.
Multiply that amount of Fluorish Iron to the volume of your tank (40) and add that amount to the tank every day. Keep in mind that the other product that you have (Fluorish) contains iron too so you will be adding about 1/3 extra iron with the Fluorish.
Verify your iron test kit too.
You will have to use Fluorish and Fluorish Iron together because Fluorish adds much needed trace elements. I can't tell you how much Fluorish to add - you need to figure that out by watching your plants. At first add what the label says but break it into daily doses. If your CO2, N, P, Fe are in line but your plants are not doing great you need to carefully increase the amount of traces (=Fluorish) that you use. If you see algae appearing that means that you are adding too much traces and the reason your plants are not doing well is something else.
I'd say forget the Fluorish tabs for now.
Lastly a positive change will not happen in the matter of a few days. If you indeed keep the nutrients in a good range you should have a much cleaner and healthier tank in about 3-4 weeks. That of course is a very general statement. Just be consistent and remember that the tank will go through phases and will eventually become healthy.