Should i use a under gravel filter?

Skip the undergravel filter. Some people swear by them but since you have a fluval you will not need to worry. Undergravel filters put oxygen "bubbles" in the water but all they do is just agitate the water, not oxygenate it. Good water flow is what releases Co2 in the water and allows O2 to enter. Since the fluval is a canister filter it creates great water flow.