In large fish, the tail reflex can be tested. For that test, hold the front part of the fish out of the water but in the normal swimming position. A fish that is still vigorous can hold its tail horizontal, but a weaker one lets the tail droop.

With a microscope (if you have one available), the skin can be examined at 10X, 20X, and 30X, but take care that the fish are not kept out of water too long, three minutes the maximum. To keep the fish from flopping off the table during any other examinations, wrap the fish in wet cotton cloth, but even then the time out of water should not exceed three minutes. Various parts of the body can be uncovered for examination and the taking of biopsy material. For that, run a scraper or knife lightly from the anterior to the posterior end of the area being tested to accumulate some slime. Take smears from the caudal peduncle, the angles formed at the bases of the pectoral fins, and the opercula (Photograph No. 37).

Mix the slime obtained from every smear separately with a drop of water on a separate microscope slide and cover with a cover slip (Chapter 11.4). Wipe the scraper clean each time you scrape it over the fish. Now examine the mounts; large parasites will be easy to see, if higher magnification is necessary, press the cover slip lightly and wipe or blot the water that is squeezed out at the sides of the cover slip. This is necessary because at high magnification a thick mount becomes too dense to see.

Any larger parasites found can be picked off with finely pointed forceps and prepared for microscopic examination (Chapter 11.6). Then obtain a smear from the gills. Be very careful because they can be more easily damaged than skin. Another way of obtaining gill parasites is to hold a glass pipette on the gill arches and aspirate; all insecurely anchored parasites will be loosened and sucked up into the pipette, from which they can be transferred to a microscope slide and then looked for at 100X.

A fecal fragment may be obtained by repeatedly applying short, light squeezes to the abdomen. Transfer the feces to a microscope slide, swirl in a drop of water, and examine at 100X. Protoopalina and worms can now be seen. To see small objects such as worm eggs and flagellates, you would need to switch to 200-400X magnification. Keep the cover slip very close to the slide (by removing water with a blotter) to prevent overlayering of microscopic objects, which makes the slide too opaque to see through (Chapter 1 1.4).

Parasites that are not discovered by the methods described above are not numerous enough to present any danger to the fish. That could quickly change, however, if environmental and stress factors change. It is advisable to examine the feces of aquarium fish periodically, even if there is no disease suspected. Wait until a fish defecates, then pipette the feces up with a long pipette before it reaches the bottom of the tank.

If the fecal thread hangs from the anus longer than ten minutes, it is limited in usefulness as a specimen, for many living parasites have already abandoned it. In addition, scavenger organisms soon begin to decompose the feces (Chapters 6.4.6 and 11.4, as well as Photographs Nos. 87, 88, 89, and 11 1 to 11 5). Many fish cannot defecate completely and for hours drag around a fecal thread--often whitish and slimy-that grows longer and longer. This indicates a serious intestinal disease, often a flagellate infection or enteritis (Chapter 6 and 9).