Nitrates,water changes,RO

[pegrochilakila]

Hi everybody im new in this hobby i been in the reef hobby for 1 year or so.
My tank is a 55 gallon some corals and fish, what i have is a yellow tang blue tang two clowns 1 blue chromis cardinal diamond goby, zoas polys and mush and a bunch of hermit, i think the already reach the point of get out of this hobby, because my battle against nitrates is a never ending situation, i did all ideas possibles, i start with the sump, wich has just live rock, then i get a RO/DI machine cus my "water" was the problem, i did 5 gallon water change every 2 days but i couldnt make them lower, cleaning removing green algae cleaning my sand bed etc etc etc, im really happy for all those guys that control their nitrates just with water changes, for me didnt work.-
all my water paramathers are good ammo nitri ph etc etc just the nitrates 20-40 i cant do nothing to lower them
Im kinda giving up but if somebody has some ideas maybe i still can walk the xtra mile
Thanks to all of you guys