why do you have to do a water change after treating your take with medicine?

Question

for example, all medications state to treat for a certain period of time, normally 7-10 days, depending on the medicine and the severity of the disease, followed by a water change. why?

Answer 1

Previous answers are partially correct, but also because many of the medicines - being antibiotics - can interfere with the "good" bacteria in the tank that convert the ammonia to nitrite to nitrate. You don't want these to die and have to cycle your tank again with a full bioload of fish, would you?

Also, the bacteria/fungus/parasites are dropping off the fish (not necessarily killed - some medications only cause them to release their hold and prevents their reproduction and by changing the water, you're removing the living organisms from the tank before they can recover from the meds) - at least this is true for levamisol (intestinal worm medication).

Some meds require a second dose, so by doing a water change in between, you are not "overdosing" your fish.

So, depending on the meds you're using, there can be a number of "right" answers to your question.

Answer 2

the water change is necessary because, just as a human only takes antibiotics for seven days, fish must only have them seven days as well. if you dont change the water, the fish remain in that concentrated medicine bath, which can cause kidney failure leading to dropsy and death.

Answer 3

when you get better you stop taking meds -- your fish should too. most meds will partially dissapate and partially build up in the tank. you want to make sure after the medications your fish has clean healthy water to swim around in too.

Answer 4

to get the chemicals out of the tank..they have already served their purpose...and they are hard on your tank.

Answer 5

fish keeps breathing your "medications"
the more it breaths the more it be come well
after its well the medication will make your fish ill because of over douse of your medications and your cute little fish will DIE in a very HORRIBLE and PAINFUL death

=)