Hi and welcome to UKAPS!
It all depends on your setup and as liveinazoo says, the species.
We really need to know a but more about your setup. I know you've added some "liquid plant food" but what was it and how much?
For your plants to thrive they need, food in the form of fertiliser and a source of carbon, along with sufficient lighting.
Can you please tell us about your setup:
Tank size
Filtration
Maintenance schedule
CO2/Carbon source
Fertiliser brand and quantities added
Lighting strength and photoperiod
Once we have these details then we'll be able to help you further.
Cheers, Dan