This is a purely theoretical question I would like the answer to BEFORE I encounter a serious fish bacterial infection again. Sometimes a water change is necessary in the middle of antibiotic treatment because of unsafe parameters, but antibiotic package directions are pretty strict about NOT doing such water changes. On the other hand it is also a bad idea to stop an antibiotic course before finishing it. So let's consider an example scenario with simple numbers. It doesn't matter if these exact numbers are realistic. The point of my question is about dosage after a water change. So let's say you are on day 2 of a 5 day course and you are supposed to add the same amount of the medicine each day but there is an ammonia spike on day 2. Let's say you did a 100% water change (not always advisable, but this is just an example). Could you then add two doses to the new water to make up for the fact that you removed yesterday's dose with the water change? The reason I am not sure about the answer to this is that I don't know if something happens to the first dose after the first day. Does some of it go into the fish or change form in some way so that doubling the dose with fresh water on day 2 would be an overdose? (I haven't studied chemistry or fish metabolisms.) If this is the case, then there would be no way to correctly dose after a water change, as far as I can see. But is the absolute amount of antibiotics poured into the water what is really important here as I suggested? Of course you can consider this same scenario with more realistic numbers. (Skip this paragraph if you are not unsatisfied by the previous paragraph or don't like math much.) For instance you could find on the morning of day 4 of tetracycline that you need to do a water change because of high ammonia. In that case, there would be 2.75 total doses of medicine in the water because of the partial water change you did on day 3. So let's say you do a 50% water change on day 4. Then there is half of the 2.75 doses remaining in the water, or about 1.38 doses. So then in order to bring the amount of medicine up to the package directions for day 4, which would require there to be a total of 3.75 doses, you would have to add back in 3.75 - 1.38 = 2.37 doses. The question in this case is the same: Is the absolute amount of antibiotics put in the water actually what is important here? Thanks to anyone who manages to read this far. Thoughts?