Thanks for your responses.
Actually, most of the time we don't know what the substance we are testing
for is, especially when dealing with fish kills. We have been approached by
suppliers to use Vibrio fischeri but the test kit is very expensive (both
the machine and the recurrent costs) and it doesn't make sense to purchase
one especially when we only get less than 10 incidents a year. These kits
require multiple dilutions and then calculations for an EC50.
Others are much simpler with a single test and % inhibition calculation.
I think they are all trying to confuse me!