Coral deaths unlikely due to nitrates

[Delphinus]

It doesn't sound like something that would help the overall health of a tank. Hard to say, now that you've found the problem (the leak) and can address it you can see if things start to rebound.

As suggested I would do the water changes, plus I would ensure I was using carbon actively (ie. power filter or phosban reactor) and changing the carbon out at a minimum of once per week.