Should I Use A Fertiliser?
Plants need a wide variety of nutrients to thrive so it would seem to make sense to use a liquid plant fertiliser to help your plants to remain in top condition, and this is often the advice you will get when your plants are not doing so well. However, in many cases the addition of a liquid fertiliser will not help struggling plants, and may even cause other problems (such as algae).
Whether you should use a fertiliser or not depends on your particular aquarium setup, and will only be of use if all other conditions for plants are met.
Although it may seem unconnected, substrate is one important factor. If your substrate is too large, water and oxygen will flow through it, which causes nutrients to be 'oxidised' and the plants can no longer use them, so simply adding more nutrients will not help as these will also be oxidised.
A good plant substrate should be several cm deep and consist of a fine grain size (but not fine sand which is too small) If you have a good substrate and strong lighting, as well as lots of plants, then the addition of a liquid fertiliser will probably help. If you only have a few plants they should be getting enough nutrients from the waste produced by your fish.