HELP! Nitrate spike - things going down fast :(

[kien]

Your nitrate readings are a bit confusing, is that 15mg/L ? The second time you recorded 0.15. Is that with different units or the same units? You said they were the same which is why I'm a bit confused.

If I recall, 15mg/l is roughly 15ppm which normally should not have caused your tank to go south. More delicate corals (like sps) would not appreciate the high nitrate levels and prolonged exposure to high nitrates may be problematic to some fish, but you said this was a spike, suggesting that your nitrates were elevated recently. I would not be so quick to blame your nitrates for current issues.

If the blenny and the coral are new then they could just be trying to adjust to your tank. They could also have been sick already, and if they were then they may have introduced a pathogen into your tank.