Plants use the most of H, O and C. Hydrogen, Oxygen and Carbon. You are dosing CO2, and the tank is filled with water (I hope).
The first 3 are the elements plants use the most of, and are called Macros:
N is Nitrogen. Most common source is KNO3, a dry powder. Also available in the Flourish product line and other liquids.
P is Phosphorus. Most common source is KH2PO4, a dry powder. Also available in the Flourish product line and other liquids.
K is Potassium. Most common source can be the potassium in KNO3, if your tank needs a reasonable amount of NO3. If your tank does not need very much NO3 you can add just K by using K2SO4, a dry powder. The KH2PO4 is dosed at such low levels that the K pretty much does not count. Potassium is also available in liquids including the Flourish product line, and Leaf Zone (also has Fe in it).
The next group are secondary nutrients.
Ca is Calcium. Most tap water supplies calcium as long as the GH (General Hardness) is over 3 German degrees of hardness.
Mg is Magnesium. Most tap water supplies magnesium as long as the GH is over 3 German degrees of hardness.
If the GH is lower than 3 degrees the most common supplement is GH booster. Seachem Equilibrium is one example of GH booster. If you need this, read the label of the GH boosters that are in your store. Make sure they do not contain sodium chloride (salt). (see more about GH below)
Fe is Iron. This is the other element that plants need in moderate amounts. You are already dosing iron. If you want to does a dry source look into the various chelated irons. Chelated means it is bound to another molecule in a way that makes it available to the plants for a longer time. Do a little research. Some forms of chelation are better for certain pH levels.
The last group is roughly a dozen minerals that plants need in very small amounts. They are almost always grouped together unless you want to go to the trouble of searching out all the individual minerals and working out the right proportions. This is not worth it! You are already dosing this as Flourish Comprehensive. A dry equivalent is CSM+B.
Fish food contains reasonable amounts of N, P and most traces. If the NO3 in the water is constantly rising, and you have to do water changes to keep it low, then there is probably also enough P and traces.
But I would dose K and Fe.
If the NO3 is constantly dropping, and occasionally hits zero or near zero, then I would dose N, P, K, Fe and traces. The EI (Estimative Index) method mentioned by Steve is a good way to get started.
GH is a combined test of both Ca and Mg. Plants and fish use both minerals. Plants use the Ca and Mg in a ratio of about 4 parts Ca: 1 part Mg. If you suspect the Ca and Mg from your tap water are not in something close to that ratio you might have to supplement with one or the other. You would need a calcium (fresh water) test to determine what is really there.
Epsom salt is a good way to dose magnesium, but has no calcium. Many places will sell Epsom salt as a way to raise the GH. This is not good. It can raise the GH so the test turns the right color, but you want the right mineral levels, both Ca and Mg. You are not looking just for the right color on the GH test.
If you think there is a problem with low or unbalanced GH, Ca and Mg, then get the separate Ca test and figure out what is really going on.
You can get all these in liquid form, if you want. Seachem Flourish is one brand name. They bottle each element separately (N, P, K, Fe, traces) You are paying for water with just a small amount of the active ingredient. That may be OK for one tank, but it can get expensive running a high tech tank with the associated high dosing levels.
I would buy one round of all the ferts to see what your tank needed and how it responded. Then buy the dry to make up your own custom blend.
You can use dry ferts. These are agricultural grade powders. They are mostly sold on line. Here is one vendor. Look at others, too. Compare what your tank actually needs with the way they are selling the ferts. Some places make up a package deal that might be cheaper if you actually need all the things in the package.