Nitrates go up for one or some or allof the following reasons plus there are more
#1 most common is over feeding
to avoid this only feed what fish will eat in less than 2 min or till the food hits the bottom then stop
#2 lazy cleanup crew due to food left on the bottom after feeding
Feed less
#3 hare algae or any form for that matter will hold detrus and that causes nitrates
clean and remove all algae possible
#4 poor water quality
do water changes often 10 to 20% at least weekly
#5 dead spots in tank allowing detrus to build up and cause nitrates
add more water movers pumps
#6 dirty filters are nitrate factorys
clean often
#7 skimmers are nitrate factorys when left dirty for long
Clean often
#8 bio balls are nitrate factories when let to get realy dirty
Clean a fiew at a time like weekly till all have been cleande and keep that way or remove slowly over time and add algae in sump instead
#9 Gravel is too dirty causing nitrate rise
when doing water changes vacume only sections of the sand NEVER all at once
#10 too big a bio load (too many fish)
quit adding and give 1 or 2 to a friend
that is all I can think of right now but they might help find the problem
Good Luck
Manny