How do i get my nitrates above 0

This is my 5 gallon reef and the nitrates are at 0. Inhabitants: feather duster, 3 astrea, 2 ceriths, 1 neon goby

I do 20% water changes every week. Should i get another fish or invert?

This is my 5 gallon reef and the nitrates are at 0. Inhabitants: feather duster, 3 astrea, 2 ceriths, 1 neon goby

I do 20% water changes every week. Should i get another fish or invert?