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So I was sitting there thinking about our next caravan holiday and started wondering, should you leave water in caravan tanks?

One of the biggest worries when I’m turning on the caravan tap is that the water’s contaminated. So I did some research: It doesn’t really make a difference. Different types of bacteria and fungi can still grow whether you leave your caravan water tanks empty, full or partially filled. The main thing is if you’re leaving your caravan sitting around and you’re worried about water quality, you should clean your tanks before you use them again.

If I keep my caravan water tanks completely full will it keep the water fresh?

You’ve probably heard that bacteria won’t grow if your tanks are completely full because:

  1. There’s no air for the bacteria to grow, and
  2. Bacteria needs sunlight.

But here’s the thing. There’s 4 reasons why bacteria can still grow when your tanks are full:

1. Not all types of bacteria or fungi need oxygen to grow

There’s two types of bacteria and fungi: aerobic and anaerobic.

Aerobic means germs that need air to grow… and anaerobic bacteria are germs that can survive and grow where there’s no oxygen.

In fact:

The most common cause of water contamination and gastro (E. Coli) is actually an anaerobic bacteria that doesn’t need air to grow.

Caravan Water Tanks Full Does Not Matter - E Coli is Anaerobic Bacteria

The most common cause of gastro (E Coli) doesn’t need oxygen… it’s an anaerobic bacteria

2. There’s oxygen dissolved in water

Fish breath by using their gills to get oxygen from water. Bacteria can do the same thing: feed from oxygen that’s dissolved in the water.

So even the aerobic type of bacteria will grow in water too.

Caravan Tanks Empty or Full Does Not Matter Because There Is Oxygen In Water - Ask Fish

Fish use gills to get oxygen from water… and so do bacteria!

3. Not all types of bacteria need sunlight to grow

It’s a big myth that bacteria needs sunlight to grow.

Think about it:

If we stick with E Coli (one of the biggest causes of gastro). This lives in your digestive system (i.e. where the sun don’t shine)

The truth is that some types of bacteria prefer the dark, others the light… many don’t care and will grow in either condition.

4. Your tanks won’t be completely full anyway

We can try to fill up our tanks as much as possible, but the reality is there’d be some air bubbles that get trapped in there anyway.


Ever notice that when you leave water in a glass for a couple of days, bubbles of air form on the edges?

Same thing happens in our tanks, bubbling up to the top.

So even if there was some truth about bacteria and fungi not growing in full tanks… our tanks wouldn’t be fully full for that long anyway.

So by the sounds of things… I wouldn’t bother filling them up.

If I empty my water tanks will it keep them clean?

Bacteria loves moisture. So it makes sense that if you empty your water tanks, then any potential bacterial growth will be kept at bay.

But here’s the thing:

When you drain your caravan’s tanks, it’s more than likely that there’s still some water pooled on the bottom. Even if there’s isn’t, there’d be some drops of condensation on the roof of the tank.

At the very least: there’s still a lot of moisture in the air.

So to make sure your tanks are completely dry, you’d need some very dry and warm weather… and to leave your van for a few days or more.

Ants Get In When Leaving Caravan Tanks Empty

[This is a comment that often pops up whenever the question gets asked about storing caravans]

Even then, you can’t guarantee there’s no water still in there.

So by the sounds of things:

It’s not worth bothering trying to empty them, because they won’t be fully empty anyway.

How long can water sit in RV fresh water tank?

The simple answer: It depends.

There’s so many things to consider here:

  1. How hot is it?
  2. Have some germs already been introduced to the tank?
  3. Which ones?
  4. How many?

So really, there’s no way you can get a definitive answer to this question.

But think about this: people quite happily have rain water go into tanks and drink it.

So there we go. I was quite surprised about this…

Full, empty, half-full, a week, a year. These aren’t really the things that make a difference.

How can I make sure the drinking water from my caravan tank is fresh?

From my research it seems like the question about should you leave water in caravan tanks doesn’t really matter.

The best way to make sure that the drinking water from your caravan tank is fresh is to:

  1. Make sure you get water from a decent source in the first place
  2. Use an inline filter when you fill up your caravan water tank
  3. If your caravan has been sitting there for a while flush the tanks and refill them
  4. If you’re concerned about the water quality then clean the tank and water system
  5. Use an under-sink caravan water filtration system too

[The inline water filter is one of the 33 Most Popular Caravan Accessories]

Leaving Caravan Water Tank Empty or Full - Less Important Than Using an Inline Filter

If you’re now keen to find out the best way to clean your tanks?

ReadHow To Clean a Caravan, Camper or RV Water Tank [Step By Step]