How To Remove Black Bits In The Caravan Flush Tank

If you’ve left your caravan unused for a while, you’ll most probably have noticed small black bits coming out of the flush and into the toilet bowl when you next came to use it. When you see it for the first time it can be a bit worrying, and you’ll be wondering what on earth it is – but it’s very common and easy to sort out.

Today we look at what causes it and how to remove those black bits from your flush tank.

remove black bits in caravan toilet

What Are The Black Bits In My Toilet?!

Well, it’s actually formed from the pink solution you put in your top flush tank. An algae forms on top of the liquid when it’s been sat in the tank undisturbed so when you pump it through the flush it breaks the algae into pieces. It can also happen when you’ve mixed too much of the solution with the water.

The algae is completely harmless – but if left for long periods can cause issues with the flush pump.

How Do I Remove Black Bits In My Flush Tank?

The good news is that this is an easy and CHEAP fix!

  • First, You’ll need to drain down the flush tank. If you unlock your cassette box and feel above the actual cassette you should find a small rubber hose. Pull the bung out of the hose and drain out as much as you can. You will then need to pump the flush on the inside toilet to make sure every last bit of liquid is out of the tank.

Make a point of doing this if you know you wont be returning to your van for a while. And never leave your tank full in the winter months!

  • Next, you want to get yourself some Milton – some might say not to use this, but I can confirm that I’ve never once had an issue with it. Fill One Cap of Milton and pour it into your top flush then fill up the tank with clean fresh water. Leave for approximately four hours.
  • Next, you need to empty the tank of the Milton solution. Same as before – empty out using the hose above the cassette and then a final drain using the internal toilet flush.
  • You will then need to refill it with cold fresh water. This should pull through all residual algae with it, leaving a clean, fresh tank.
Milton Sterilising Fluid 1L - Pack of 2
  • Developed to sterilise baby in only 15 minutes
  • Kills 99.9% of germs
  • No need to rinse
  • Solution works for 24 hours

And that’s it, job done!

You Might Be Interested In: Thetford Cassette Toilet: Common Faults And Easy Fixes

How Do I Stop The Black Bits from Coming Back?

The steps above should stop the algae from reforming, growing back and contaminating your tank and toilet bowl.

Be respectful of a fact that you’re using Milton solution, so don’t just throw it across the fields or grass, make sure that you dispose of it down the designated drains responsibly.

  • Always make sure to always drain the tank down whenever you leave your site. It doesn’t matter how empty it ‘might’ be – try to make sure it is completely empty. This can also help to prevent any potential damp issues.
  • Also make sure you run the flush pump on the toilet because even with the tank being empty, there’ll still be a little bit of fluid left over in the actual pump, so make sure you press the flush and completely empty the system.
  • When you next return to your caravan and make up the pink solution, make it slightly weaker than what the manufacturer’s guidelines state.
Why we don't use pink fluid in our caravan toilet flush.

When you make it weaker, it’s less likely form algae in the future. Also, you don’t ‘actually’ need to use the pink fluid – I’ve seen many people just use plain water, and a couple of people have actually suggested to me using Zoflora in the flush tank!

We do still use the pink solution, the one that we’re using at the moment is the Thetford one. We’ve used other manufacturers in the past – some are better than others but they can all cause this problem. So please don’t think that this is something to do with one manufacturer, it’s just a chemical by-product found in all of them!

Thetford Aqua Kem Rinse Concentrate
  • Contents: 750 ml.
  • Sanitary concentrate for flush water, additive concentrate for fresh water tank.
  • Small and powerful: A powerful, highly effective, two and a half times concentrated sanitary fluid for the waste-holding tank of cassette and portable toilets.

That’s it – it’s as simple as that. We do this once every three months or so, especially when we use the toilet often as it just makes sure everything is nice and clean.

If you are still having some issues you could half-fill your tank with water and Milton with some dry foam. The agitation when towing will loosen up any residual algae.

I hope this post helps you if you are having any problems with black bits in your toilet flush. If you have any further suggestions you want us to add to this post – drop me us line at and we’ll be happy to add them here!

Thanks for reading and happy caravanning!

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