We have given the highest dose of 2000 mg/Kg body weight. The animal was left for 24h and there was no symptoms observed. The fish was healthy and so we took 1/10 as one of the dose. Now how to decide two other doses for screening.
Try with multiple (assume identical) subjects to conclude. 2g/Kg body weight is quite low for chemicals having low or moderate toxicity. See TLVs of various chemicals from ACGIH website for further study/guess.