Just wanted to include a point.
Before purchasing Medicines, read online about the optimal water parameters for the medicines. Do not add medicines to poor water conditions, it could create a toxic mess for the fish. You will want to make sure the water is in optimal condition before adding medicines.
Take
Tetracycline, a common and often recommended antibiotic, however it will not work in every tank. I did not find this info on the package in the store, however on line I found
http://www.aquariumlife.net/articles/aquarium-care/104.asp (and several other sites) this antibiotic is not effective in water that has a PH of 7.5 or higher. My aquariums have a PH of 7.6 or higher.
So if it works in someone else's tanks it could be their water parameters are within the chemicals requirements to be effective. Thus this is why it works for one person and yet does nothing for someone else.
So do not get exicted and dump medicines in at first sign of sickness, take your time and figure out what is going on. test the water and observe their behavior. Also this is another argument for purchasing the test kits, know your water, and know your fish.
Hope this helps.