Need advice: Nitrate at 80, but ammonia and nitrite at zero?

Hello. I am a new to salt water and just joined the forums. So far I love test and caring for my 125g salt water tank.

So I purchased the tank used (1) month (2) weeks ago. I had it running since day one. I used transferred the same live rock and gravel from the previous owner, but it did dry up during the transfer. The one thing that was completely new was the salt water. I did have about 6 fish in the tank since day one since those also came with the fish, which thankfully are still alive till today.

Anyway, after waiting a month to finally do a water change and researching about testing my water, I finally started to run tests.

Here are my levels as of today:

ammonia: 0 ppm
Nitrite: 0 ppm
Nitrate: 80 ppm
Phosphate: .20 ppm
Calcium: 440 ppm
PH: 8.1
Salinity: 1.021
Alkalinity: 10dkh

Since I went past the one month part, I added a few more fish making it a total of 12 fish and I also added 5 corals and 2 soft corals, and one anemone. so far all are doing fine, but from my tests I see that nitrate is super high compared to my other test. I actually did my first water change at one month one week, and my second at one month two weeks. (15% each time).

all levels stayed about the same. Ammonia and nitrite went to zero, but nitrate actually increased from 20 ppm to 80 ppm. I did feed my fish a little too much the last week, but I assume with the water change it would improve. Anybody have an idea of whats going on and how I can lower the nitrate, or should I wait?