Unless the tank walls are so coated with the animal fats in waste that they need cleaning
, it's not necessary to fill the tank to rinse it out:
1. empty the tank via pumpout or (if you can take the boat
out beyond the "3 mile limit") the macerator pump.
2. put enough water into the tank via the deck
pumpout fitting--'cuz that sends the water into the tank at the bottom to stir up the sludge and hold it in suspension so it can be pumped out or dumped--to cover the bottom to a depth
of 6-8". Pump that out...repeat, repeat until you're pumping out clean water.
It's ok to use salt water
for this, so if you have a washdown pump and macerator pump, and can get out to sea, use the washdown pump to supply the water...when the depth
reaches 6-8" turn on the macerator and let the water continue running till the tank is clean.
If you still need clean the tank walls, put at least a gallon of liquid detergent in the tank and fill with water this time. Put about 10 lbs of ice cubes in the tank too...go out and tack a lot. Dump the detergent solution...fill the tank again with clean water. Repeat if necessary.