Did you have a look at this ?: https://www.oecd-ilibrary.org/environment/test-no-203-fish-acute-toxicity-test_9789264069961-en

DMSO is also an option as a solvent.

• Dissolve the Profenophos in the solvent at a round number near the solubility limit to make a stock solution.
• Make serial dilutions in water and add to a fixed volume of water with 5 fish in it.
• As a start use 1/10 dilutions to perform a ranging test to identify roughly between which two values the mortality occurs. Start testing the lowest dose and work up.
• Next make a linear series of dilutions and do a single replicate to refine the range between the no observed effect and total mortality. Identify the doses between which 16-84% mortality occurs and make a minimum of 5 doses equally spaced between that range.
• Perform 3 replicates with an N of 5 or greater per dose per replicate.
• Convert the doses tested to their log values and perform a linear transform on the % mortality (probit or logit is good) and then plot dose along the x axis and the transformed mortality along the y axis.
• You can then perform linear regression to identify the dose that cause 50% mortality (LC50).

Try to make a biological window and find exponential mortality.