Fish blood appears colorless under a microscope, what does blood appear red when observed w/ the naked eye?

Answer 1

Blood appears red because of the oxidized iron within hemoglobin molecules. The fish blood may appear colorless if you are using a prepared-mount slide that is old, rather than a freshly prepared slide. The blood's color may fade as it dries.

Answer 2

All red blood cells appear pale when seen under the microscope, not just fish blood cells. Since only the hemoglobin molecules are red, they must be far apart from each other in the blood cell. It is just like diluting a blood sample with buffer. You do not see anything red when the blood cells are far apart from each other.