Otherwise, there is the chance that smaller parasites may be missed because they are hidden in the mucus. Don't jump to conclusions The second, more common, potential problem is simply jumping to conclusions.
It is just too easy to spot the most obvious parasites, for example skin and gill flukes, and conclude that these are the problem.
So here is the original article i got from the ‘Goldfish and Aquarium Board’.
I must stress that i did not write this article, i'm just sharing it.
The mucus sample is then smeared onto a clean microscope slide along with a drop of pond water.
Never use tap water as any residual chlorine could kill any parasites that are present!
In the first instance we want the fish to be still enough to do the scrape properly, while at the same time avoiding damage to the epidermis.
With two people, one holding the fish and the other taking the scrape, it is possible to sample smaller fish and docile larger ones without the need for sedation.
A mucus sample can be taken from the gills by gently inserting a cotton-bud under the operculum and rolling it over the gill filaments.
The sample is then covered with a cover-slip and examined under the microscope, usually low-power, for the presence and number of parasites.
I would normally take at least two samples, from different sites, from each fish being examined. There are certain considerations if the examination is to yield useful results and avoid causing damage to the fish being examined.
For accurate results it is important that the sample is prepared and examined as quickly as possible.
With care it is also possible to take a small sample of gill using fine scissors to take a small biopsy from the lamellae tips.