Since medication solubility and activity questions are
important, the following may help us determine whether
the proposed fish dose is reasonable:
1) The Merck Index lists the water solubility of Flagyl,
which is the commercial trade name for Metronidazole, as:
10 gram per liter at 20 C.
Solubility at the fish dose under consideration
of 1.5 gram per 40 liter should not then be a problem.
2) At the Searle web site
one finds that a microorganism is considered susceptible to Flagyl if
it stops growing at or less than 16 microgram Flagyl per milliliter.
This is also called the minimal inhibitory concentration (MIC).
Many Flagyl susceptible organisms have a MIC of 1 mcg/ml.
Since 1.5 gram per 40 liter is equal to 37 mcg/ml
then at least the we can say that the fish water
is at more than double the Flagyl MIC.