If you're only talking about critters then standard household bleach works. Use the table below. If you want to also clean out any debris it depends on if you have a clean out access plate. If you do,,, I'd stick a pressure washer end or a hose nozzle on JET in the tank and blast it loose and then pump it out. Repeat a couple of times.

We try not to have any sediment in the tanks by using the boat. There' nothing like beating into 25 knots of confused seas for a few hours to loosen up any junk in the tank.

From this website http://www.csgnetwork.com/h2oemergencypurifycalc.html
Ratio of Clorox Bleach to Water for Purification

2 drops of Regular Clorox Bleach per quart of water
8 drops of Regular Clorox Bleach per gallon of water
1/2 teaspoon Regular Clorox Bleach per five gallons of water
If water is cloudy, double the recommended dosages of Clorox Bleach.

Only use Regular Clorox Bleach (not Fresh Scent or Lemon Fresh). To insure that Clorox Bleach is at its full strength, rotate or replace your storage bottle minimally every three months.