Even the best kept aquarium will eventually have a sick fish. Pathogens are sneaky and can come from unexpected places, but typically unfiltered water or a new “friend” is to blame. The quick fix solution for most people it to run down to the local fish store and grab an antibiotic that covers the symptoms. With marine aquariums however, you need to remember that nitrifying bacteria are bacteria too and antibiotics don’t know the difference between the bacteria we are trying to kill, such as cyanobacteria or Flexibacter columnaris and many others vs. the good bacteria (probiotic bacteria) that we are trying to sustain. Therefore when treating a sick fish with antibiotic medication, it is recommended to use a hospital tank if possible and avoid adding an antibiotics to your main display aquarium if at all possible.
If you are in a situation where it is not possible to use a hospital tank, such as having multiple sick fish or you simply can not catch the sick friend, you will need to take steps to protect the good bacteria.
You will also need to make sure you are changing 5 – 10% of your water once a day for the span of the use of the medication. Not only will this help remove the accumulated toxins from the water little by little, but it will also allow the medication to work more effectively. Dissolved organics in the water column can render the medication less effective and in some cases can increase the toxic side effects of the medications. By continually changing the water, you are lowering the level of dissolved organics, allowing the medications to work more effectively.