About designing a lowpass filter ?

while doing Am demodulation i am sending wave with frequencies 1Khz,4Khz,6Khz which are created after mulitplying modulated signal with carrier.I need to passthrough this signal to lowpass filter to get only 1 Khz signal .My Fs=25000,Fp=1200,Fs=1500,which is the best filter so i can use.Becuase what ever filter i use i am getting harmonics.How to remove these harmonics...beucuase i am expecting a pure sinusoidal signal becuase i am doing everything in matlab only..i have designed filter using FDATOOL...

i got stucked here for last few days..please help me

Thanks in advance..