Hey Boatman!

A 50/50 mix of bleach and water works great. Fill the tank with this, then pump-out at a shore station--not into the water, as bleach kills just about everything.

Other tips I've learned, running big boats for paying owners:

1. After the tank is empty, as two cups of water softener, available in the laundry department at the grocery store, dissolved in hot water. The softener prevents solids from sticking as much to the tank and lines.

2, Install a water filter on the intake line--if you fill with a hose, there are hose filters. This is will keep out some of the critters and minerals that can start the smell.

3. Change the hoses regularly. Frequency depends upon use, but I;d say 3 years, 4 max, swap out for new hoses. Most of the odor comes from here.

4. Eliminate as much as possible any bends, kinks, low spots in the discharge hose runs.

G'Luck
Kevin