The entire surface of the fish should be thoroughly examined including the oral and opercular cavities, the nares, gills, and fins.
If there is a time restraint or a large number of fish are to be inspected, most external parasites can be detected by examining a mucous scraping from a number of sites along the body, including the head, pectoral and caudal fins. The use of a hand-held lens can be useful in examining the surface of larger fish.
A mucous smear can be made by touching a glass or cover slip onto the side of the fish or mucus may also be collected with the non- cutting edge of a scalpel and wiped onto a slide. A fresh preparation may be mixed with saline and examined with a compound microscope, using a 10 to 40X objective. The use of phase contrast may be helpful for the recognition of protozoans. When preparing a fresh smear, care must be taken when placing the cover slip onto the material. Artifacts can result from excessive pressure. If necessary, tap water may be substituted for saline in any of the preparations given in these procedures. However, the use of saline may be critical to maintain a suitable osmotic environment for certain parasites, and is the medium of choice.
If there is enough saline on the slide and the cover slip is lowered properly, no air bubbles should be present. Excess saline can be removed by touching a paper towel to one edge of the cover slip. In the event there is insufficient saline, it may be added at the edge of the cover slip.
Source: General procedures for Parasitology by Jack Frimeth, Onatrio Minstry of Natural Resoures, Fish Health Lab, Ontario, Canada