So my cycle seems to have stalled at high nitrates. Help please? :D

[Enigma]

You need to test your new saltwater for everything before doing a waterchange: but especially ammonia, nitrites, and nitrates. You need to be sure that you've got zeros for all three in your mixed water.

If all three are zero, that eliminates one possible source.

If all three are zero . . . Something is decaying in your system: beyond what your bio filter can handle. It could be gunk in your filter, over feeding, dying critters . . . ?