yes the more the fish the higher their death rate is. Lets do extremes. 1 out of 5 fish die, and 1 out of 500 die. the rates are .20 or 20% and .002 or 2% percent. The same number of fish died but the rate changed because the total number is different. a controlled experiment should have the same variables nonetheless except for the manipulated variable, so even if they were the same rate, unless the number of fish is your manipulated variable, you should have the same number of fish.

Make sense?