I suggest counting the following... take a small segment of your tank, (Just a general random one, not one particularly lacking them or having an over-abundance) and count the amount of seamonkeys in that square area... then measure the length and width of that segment and the depth of your tank... this is the number of sea monkeys present within that volume of tank...
MATH FINALLY COMES TO THE RESCUE!!!
Because the sea-monkey population will be evenly spread throughout the tank (I don't know how big they are but I figure they're pretty small) you can then measure the width, depth and height of the tank area filled with water. Now... Just use the following formula:
Total_Volume * (Sea_Monkeys_In_Small_Volume / Small_Volume)
This should give you an approximate number of sea monkeys for the entire tank by only counting a small number every time... In fact you can change the size of your distance measurements and count even less sea monkeys as their numbers get large... You sea monkey population as a function of time should resemble something called the Logistic curve.... that means that it should rise rapidly like an exponential (e^x) and then fall off as the population runs out of food leveling off at the end... I did this with zombie population growth... it was fun!!!
(Ironically if you have a predator prey system with something to eat Sea Monkeys, a sine wave looking thing results O_O, that's why enviroments look the same year after year... their populations are simply waving back and forth, you can actually describe these results using differential equations pretty cool (boring) right
. )