Amongst the newly discovered compounds that are "silver bullets" in ridding certain pet-fish diseases, Metronidazole, aka Flagyl is a shining star. With careful use it can quickly eradicate some of the deadly and debilitating protozoan complaints of freshwater and marine fishes. Particularly useful for hexamitosis (the causative organism in many cases of Head and Lateral Line (Neuromast) Erosion, or corrosive syndrome, and many gut infestations, Metronidazole does have a definite downside. It's prolonged or too-repeated use results in the death of stock from renal (kidney) failure.
Principally for internal (lumen) protozoans and the flagellate commonly associated with HLLE (Hexamita, Octomita), though is promoted for use against other protozoans (Cryptocaryon, Ichthyophthirius). These last are better treated in other ways. Some groups of marine and freshwater fishes might/should be treated prophylactically... coming either directly from wild-environs or culture facilities known to harbor standing parasite fauna. Signs of neuromast destruction (HLLE) are open-pitting of the head and lateral line pores of fishes... Internal flagellate complaints manifest themselves in lethargic behavior, sitting on the bottom, bloating, slow, labored breathing, a lack of feeding...
First off, know that Flagyl is only useful in the amounts that "get inside" your livestock... Though marine fishes drink "proverbially like fish", freshwater do not... and in any case, the only efficacious means of administering Metronidazole is through food. There is a further problem/issue of water solubility. In the process of mixing Metronidazole one must blend it well with water ahead of time... as this material is not very water soluble. Soaking frozen or live foods in 1% solutions for a few hours in a refrigerator is a very good idea. Actual dosages are best at about 0.25% Metronidazole fed at a daily rate of 1% of body weight. Feed just once usually, no more than thrice.
For marines, long-term baths (a few hours) in more concentrated solutions may work... 5 mg of Metronidazole per liter (19 mg/gallon) administered daily for three treatments. Of course you must provide aeration during these baths.
Some authors advocate tossing Metronidazole in the system water itself.... stated dosages range from 6.6 to 25 mg/l of 25-95 mg/gallon... either daily or every other day for three treatments total.
Metronidazole Dosage 4/27/10
As stated, not safe to use continuously or repeatedly.
Use With Other Chemicals:
Safe to add, treat while in the presence of other compounds.
A good deal of "anomalous" imported (from the Far East) livebearer and anabantoid losses, and mormyrid, polypterid losses from Africa exporters, and wild discus and angelfish from South America (from intestinal flagellates)... have been solved with a single dose or dip with Metronidazole... It is also the treatment of choice for "Hole in the Head" (and side) "Disease"... along with restorative measures to improve water quality and adequate nutrition. However, all should be warned and aware that Flagyl is toxic and accumulative in its toxicity. The author has seen quite a few valuable (expensive) nishikigoi (koi carp) killed by careless administration of this anti-protozoal. I do wish there was some compelling way to have the trade itself pre-emptively check, quarantine, and treat to avoid the transmission (on to consumers) of such easily defeated pathogens... sparing end-users the dangers of using such compounds, and avoidable losses.
Management of Hexamita in Ornamental Cichlids http://edis.ifas.ufl.edu/VM053
Andrews, Chris, Exell, Adrian and Neville Carrington. 1988. The Manual of Fish Health. Tetra Press, NJ. 208 pp.
Herwig, Nelson. 1979. Handbook of Drugs and Chemicals Used in the Treatment of Fish Diseases. Charles Thomas, Illinois. 272 pp.
Noga, Edward. 1996. Fish Disease. Diagnosis and Treatment. Mosby-Year Book, Missouri. 367 pp.