If you are talking about Flourish comprehensive, I use that for the Micros.
I also use Tetra Pride for my iron.
The light level is not the only contributing factor. How much plants and fish you have are also included as factors. With a fairly light load of plants and a good number of fish the plants can get most of what they need from fish food/waste. This is often short on
a couple of nutrients though.
You can wait and see if any deficiencies develope or you can just get the three nutrients that you are short on or lacking completely. There are many sources for these ferts but this one is from someone on here and is just as if not cheaper than from an other source.
The ones you don't have are the Macros plus iron. There is only two nutients
needed for the Macros, KNO3 and KH2PO4. Sometimes when excess nitrates happen
from the KNO3(the NO3 part of it) combines/w the nitrates from the cycle and
people substitute K2SO4 in equal amounts for the KH2PO4. But usually half of each.
The iron can be gotten from Tetra Pride or the iron found on that site and should not be dosed onthe same day as the Macros are dosed.
I believe you have the same level of light in one of my tanks, but mine is a 10g.
I use no Excel in there and dose(the Flourish and Pride on the second day) 1/32 tsp each of KNO3 and K2SO4 so just to insure that you have some in your tank you could do 1/4 tsp of each without chance of build up if you do water changes each week.
One dosing system uses 50% water changes each week to keep from having a build up but the amount of doses I suggested is far less than their doses so a 20-25% water change will work if each week on your tank(depending on your fish level also).
I will stress again that with your light level you may not need any added ferts but
likely will need a little of them.
On the KH2PO4 only a very small amount is needed. 1/32 tsp is a good place to start unless deficiencies show up.