skyhigh nitrates and zero fish

[e46er]

ok so last week before i removed my fish for QT my nitrates were 5-10 i did a 3 day blackout followed by a 50G or so WC and now my nitrates are like 40-50ppm
was this solely from the algae dying? i mean i introduced my first fish first week of january and ive gone thru 4 buckets of salt so im not skimping on WC at all but im starting to feel this is rediculous amounts of salt im going through

i am running a phos phate reactor and have chaeto in my sump with a light thats lit 18 hrs a day
will this eventually start uptaking the nitrates becuase i cant do 50 gal WC changes daily