Leave Water Tanks Empty or Full?

[BeautifulDay]

I want to prevent algae growth in the three water tanks and lines of my Catalina 445. My boat is in a warm climate (Southwest Florida) where high humidity and temperature can play havoc with both water and diesel fuel. I try to keep my fuel tank full and use an appropriate additive. So far, this has avoided algae contamination problems experienced by other local sailors. I recently came back from a three week sail to the Florida Keys. During the trip, the water in the tanks was stale... despite my previous use of a small amount of unscented Clorox Bleach. The boat is normally used for day sailing. As a result, the water in the tanks does not typically get drained through regular use and thereafter refilled with a fresh supply. At the conclusion of this recent trip, I returned with empty water tanks and the current dilemma:

Do I leave the water tanks empty until I get ready to leave on the next passage, or fill them up now?

Leaving the tanks empty will enable me to day sail a lighter and faster boat. It will also translate into a very fresh water supply on my next voyage. But, on the other hand, I am concerned that an empty tank doesn't mean there isn't some residual water in the very bottom of each tank. There could also be residual water within the hot water heater. Will the empty tanks provide a lot of room for oxygen to infiltrate the residual water in the darkness to breed some nasty algae. After all, isn't that why we are told to keep the fuel tank full? Although I have read about the benefits of keeping the fuel tank full, I have never seen such advice in connection with the fresh water tanks.

Your comments, suggestions and advice are greatly appreciated!

[billyruffn]

I think I'd leave them empty. If you have a way to flush water through the tanks before using them it would help a lot. If you can't easily flush them, at least you'll dilute the any foul contents with a full tank of fresh water. Keeping the insides of tanks and water lines "in the dark" will help reduce algae growth. A lot of the water hose sold today is transparent which doesn't help.

You keep fuel tanks full when the boat is in storage to prevent condensation from putting water in the fuel (which is a big contributor to the algae problem). Full tank = little air in tank = less water vapor = less condensation. Another thing you can do is put a ball valve in the fuel tank vent line and close it when the tank is not in use. That helps lower the turn over of air in the tank.