Plant growth depends on more than just fertiliser, though if you want value for money, the Seachem range of fertilisers is superb. The general purpose one (Seachem Flourish) is ~£7 for 250ml online. It may seem expensive but I used to use it and only needed ~0.3ml daily to my 15-gallon tank. This would give me a whopping 2.25 years worth of fertiliser for £7!
Other growth factors involve lighting levels and periods, and whether you are adding CO2.
