High nitrate only!

[fresh]

Hi all,

I just found out that my nitrates are sitting at 80. What is puzzling is that both ammonia and nitrite are reading 0! Other readings: salinity=1.0255, KH=11, PH=8.

The tank is a BioCube 14 with:
Softies and lps + One damsel + 2 shrimps + 3 snails + 1" arag sand bed + 20 lbs of lr.
all looking healthy and active. They all ate yesterday. I only feed once every 2 or 3 days.

The tank is almost 2 years old, but was moved 3 months ago.

Only one thing is doing bad: 3" cap coral. This was a test to see if I can keep SPS with stock lights, and obviously not. It has been fading in colour for the last 2 weeks or so. It is almost completely white now.

I have already started preparing water for a water change. I will do a 3 gal water change tomorrow.

Questions:
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What I would like to know is if you have had a similar experience where nitrates in your reef were that high? And if things can continue to look healthy if nitrates are that high? why would nitrates be high, and ammonia and nitrite are reading 0? Can the dying cap cause this on its own?

Any advice and input is appreciated.

Thanks
Fresh.