Hi Dubbo,
No - Flourish excel shouldnt be needed if using CO2, but you mentioned liquid fertilizer which is something completely different.
As you have probably already read, you need lighting, nutrients, and CO2 for your plants to grow well. I would suggest adding liquid fertilizer to the water (PMDD etc) or modifying the substrate (would require a rebuild) to include something like Daltons aquatic mix.
Also - what temp is your water?