I think you need to take two steps back here and not make any forward for awhile. First and foremost that NO2
reading needs to be zero. Any nitrite is bad news and signals major imbalance in the aquarium.
Also, saltwater cycling is a rather slow methodical process. Most salties allow their tanks to cycle only adding one or two small fish after about one month of cycling. Then adding a few smaller fish after several more months making inverts some of the last animals added only after the aquarist is certain the cycle is complete. Inverts are even more intolerant to unclean water then fish.
Also adding a canister filter on your new 75 gallon aquarium may just create a nitrate factory. I think you would have far better results using only live rock and an above average protein skimmer.