were these new fish or fish from an existing system?
how long have you had the tank running for?
how long have those tow fish been in the tank?

The copperband was from a LFS in Calgary, they had had it for three weeks. The Blenny came from a different LFS in Calgary

This tank has been up and running for around 4 months in the current setup

The copperband was in the tank for just over 2 weeks, the Blenny almost 4

any signs of cyanide catching on the copperband?
I think the easiest signs are clouded eyes and pinkish cheeks (or the area slighty behind where I imagine a fishes cheeks might be), like it's blushing?

I noticed a bit of a cloudy eye a few days after I added the fish to the tank, but seemed to clear up after anothe couple of days. Wasn't aware of anything else.

Today the tank looks good, Mandarin is still nice and fat/healthy looking. Foxface and Kole are out and active as usual.

Wierd

hmmm.
Seems more likely it was a fish issue than a tank issue.
I suppose all you can do when your params are normal is watch and hope...