can tetracycline harm fish

Question
Hollie, I have a 120 gallon freshwater tank, it had 17 2 1/2" to 3 1/2" cichlids, 2 large blood parrots, several small parrots and various other fish. One of the large blood parrots developed a white growth on its head and gill plate, I could not identify the disease. An aquarium "expert" at a pet store recommended treating the tank with tetracycline for a 5 day period and then changing the filter charcoal and doing a 50% water change at the end of the 5 day period. Starting on day 3 of the treatment my cichlids started to die. This is about day 10 since I started the treatment and to date I have lost 13 of the 17 chchlids. NONE of the other fish have been affected. Could this be due to a reaction to the tetracycline?
Thanks, Tom
P.S. The blood parrots white spots have disappeared.

Answer
While Tetracycline does cure, it also kills the nitrogen cycle bacteria. So when you medicate with this, you need a hospital tank with frequent water changes.  Medicating the large tank most likely caused a spike in ammonia or nitrite due to the loss of good bacteria.  Check your levels and see if this may be the case.  Let me know if this helps.