In my experience you should only use fertilisers at all if you are showing good growth. All too often people add liquid ferts which aren't necessary - I've just done it myself and payed the price in an algae bloom - although a minor one. Generally unless you have a lot of light (over 2 Watts per Gallon) and inject CO2 liquid ferts are not required. There's enough nutrients in the fish food/waste and tap water (and hopefully your substrate).