I am not sure what products are available in Singapore, but here is a quick rundown on what plants need.
Macros are fertilizers. Nitrogen, phosphorus, potassium.
Plants use these in large amounts. If it is a heavily stocked tank, lots of fish, then the fish food can supply the N and P. Fish food does not have so much K. If the tank does not have many fish or shrimp, so you are not adding foods, then you are not adding N, P or K in enough quantities to feed the plants.
Secondary nutrients.
These are minerals like calcium and magnesium.
Test your water with the GH test. If the GH is higher than about 3 German degrees of hardness there is probably enough Ca and Mg in the water, do not bother to dose. If the level is too low you can dose GH Booster, or Seachem Equilibrium.
Iron is worth adding, unless your water already has some. It is usually not present in fish food in enough quantity. Seachem has some iron in Comprehensive, and bottles iron separately.
Micros:
These are minerals the plants use in very small amounts.
Seachem Comprehensive is mostly micro nutrients. Fish food also supplies a fair amount of most micros, but not usually enough for a high tech tank.
Carbon:
Plants use more carbon than any of the other items listed. Supplementing the carbon is very important in a high tech tank. Excel is a good carbon source for small tanks.
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Fertilizing the plants with just some of what they need, and not others is like you trying to live on limited food variety. You just do not get all the vitamins and minerals you need.
Plants that do not get all the things they need will not grow well. They will show deficiency symptoms. For almost all the nutrients the first symptom is slow growth. Then they stop growing. It does not matter if you supply plenty of one material. If there is a lack of something else they cannot grow.
When you supply a small, steady amount of everything the plants will grow.
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Here is what I would do:
Look at what you are already supplying from fish food, water changes, and fertilizers you already have. If it is enough of certain fertilizers, then do not add more of that, just keep on doing that.
Buy the material the tank is lacking.
Here is how I did this for low tech, heavily stocked tanks:
Fish food supplied N, P, micros.
Plants showed deficiency in K and Fe.
I added K, Fe and C.
Plants grew faster, started needing N, P and Micros in amounts greater than I was supplying with fish food, so I started adding those.
