The live rock structure looks nice and the sand depth looks great. The wonderful thing about this is that you have a very effective method of biologically breaking down organic waste and of denitrification, which will reduce your nitrate levels to zero in a properly set up system.
Which is the first thing we need to address. Why do you have a biofilter? The end result of biological filtration with a biofilter is the production of Nitrates. You al4ready have a method of ammonia and nitrite removal, and a method which does not harm the system with nitrate input. Your biofilter is HARMING your aquarium.
Is this a 20 gallon tank? I can't tell from the pictures. We really need to see the entire system. Post several pictures, including equipment, sump, drain lines, etc.