What causes NITRATES to be high?

doing the cleaning, this time I removed all ornaments, and plant pots and really went to town on the vacuum. Got my nitrates down to 8 :headbang2:
Added some seachem di-nitrate to filter.
So looks like the gravel was a problem. Usually I dont vacuum that heavily, as there are alot of plants n ornaments in there.
That was a total of 4 wc since yesterday. I removed 25-40%, refilled, with added prime, then removed another 25%. Left it overnight, then did two more 25% changes.
Lets hope it stays there!
thanks heaps for the comments everyone, I guess I forgot how much the gravel can affect things, have always been waiting for the nitrate, never had to get rid of it before!
 
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