rapid breathing

Hello everyone. I am very new to this hobby and I am about to lose all of my fish. I currently have a 50 gallon tank with 4 fish (i had 5 yesterday but one sadly passed). I have been keeping up with water changes but I have been dealing with cyano for the past few weeks. On sunday, I did a...

This morning I couldn’t find him my [lawnmower blenny ] and once I did he was hiding in an unusual place under the rocks and (white in colour)which is not his usual colour and he appears to be (breathing rapidly) ... is he dying ? What can I do to help at the moment ? And what is wrong with...