Debra, when you say pump I will assume you mean an additional filter for both filtration and to hold media to trap debris. It's possible you will have enough filtration when the fish are small OR if you stay on top of filter cleaning. The only way to know if you have sufficient filtration is to monitor your water parameters as the fish mature and produce more waste.
A power-head only provides water movement in the aquarium unless you have the type that allows adding a sponge filter to the unit.
Many fish keepers use either power filters (HOB style), canister filters, a combination of the two or they use a sump.