I'm currently dosing Green Brighty Neutral K, Green Brighty Mineral and occasionally Green Brighty Iron.
There is one basic problem with these fertilizers. Other than iron, we have no idea what is in them. The manufactures doesn't list the ingredients on the WEB or on the bottle. If your fertilizers don't include just one nutrient you can easily get very slow growth. And often with very slow plant growth you may not see deficiency symptoms. This makes it very difficult to impossible to determine what the problem is. If is is not listed on the fertilizer bottle and you cannot test for it, assume it is not in your aquarium.
Flourish comprehensive based on the ingredients list look good but it has very low levels of the N, P,Ca, Mg, Zn, and Cu so again it might not work for you.
Most tap water has calcium sulfur and chlorine. As a result most fertilizer don't include these ingredients. Typically this works well. However most doesn't mean everyone. You water might be soft enough that you are not getting enough of something that is not in the fertilizer.
If your water is this soft the first thing I would do is to add a sulfate GH booster to the water. Seachem Equilibrium is a good choice and the ingredients and concentration of the elements it contrains are available. If you add enough of this to increase your GH by 2 degrees you should satisfy the Calcium, magnesium, potassium, and sulfur needs of the plants. You can then add 10ppm of nitrate (KNO3 is commonly used ) and 1pppm of phosphate (KH2PO4 is frequently used) you have now satisfied all the macro nutrients N, K, Ca, Mg, P, S.
With your test kits showing 20ppm Nitrate your can probably avoid the nitrate fertilizer. But if it drops to 5ppm or less add nitrate.
As to the micro nutrients plants Need (Cl, Fe, B, Mn, Zn, Cu, Mo,) the easiest way to add this is with a good micro fertilizer. Many fertilizers don't contain Cl. This can easily be addressed by adding table salt (NaCl) 5 to 10 ppm in the tank should be enough to provide the needed Cl. Some fertilizers claim they are safe for invertebrates (shrimp and snails) because they don't include copper in the fertilizer This is OK as long as you have copper in your tap water. But if your home doesn't have copper pipes or you have very soft, RO or Distilled water your plants will grow very slowly. I would avoid any fertilizer that say it doesn't have copper. I have looked at a lot of aquarium fertilizers and have not found any with dangerous levels of copper in it.
I don't know what is available in Hong Kong but if you can get a bottle of thrive and Seachem Equilibrium you have a very good chance of getting better plant growth.
Also you might find Rotalabutterfly.com a useful web site. It has fertilizer calculators and other useful information.