Tolerances/Dangers:
As stated, not safe to use continuously or repeatedly.
Use With Other Chemicals:
Safe to add, treat while in the presence of other compounds.
Cloze:
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.
Bibliography/Further Reading:
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.