Nuevo Flush Tank.

I have a persistent problem with a black mould which keeps appearing in the flush tank of the Thetford system in my 2008 Nuevo.
I have bleached it and that worked but not for long. 
I think that I might have to dismantle it but I have no idea how to do this and I would rather not.
We use the van every week for two nights and use the pink fluid with every fill.
Any suggestions?

I use pink stuff (not Thetford) and never have the black bits but do run the tank until it's empty after every trip.

Thanks Paulmold,
I have never run the tank dry except when bleaching it.
I'll give it a go.

I have never had the black bits but I don't use pink. I put biological Persil in the flush tank. I also stopped using Elsanol in the cassette several years ago after over 40 years. I was a bit nervous initially but prefer it hugely. The detergent keeps the bowl slippy and emptying the cassette seems easier. The smell is different but no more unpleasant and the waste is environmentally friendly. I suspect that the lubricating properties are also better for the pump and other moving parts. I put a more or less full measure in a fill from dry and guess when topping up. The colour of the flush confirms if it's right.

Used the pink stuff for years and suffered the black bits problem, drained the whole lot out and we now just use plain water with a few drops of Thieves essential oil which smells great. We also have a small spray bottle for use in the pan.
It is expensive to buy but a little goes an awfully long way.

Thetford Pink was notorious for contributing to the black mould.
We used to us Morrisons Laundry liquid, which did the trick and had a lovely aroma.

Similar to Hog, in my opinion it's a complete waste of money adding anything other than plain water to the flush tank. Vans that take the feed for the flush from their main water tank have no additives, so what's the difference?
To eliminate smells this stuff in amazing and lasts for years.
https://www.poopourri.com/

No problems here with my pink stuff

The black bits are not mould, they are solid bits of the pink stuff that have flaked off the interior of the water tank. Basically as the pink stuff evaporates on the sides if the tank and it's contents it leaves behind a molecular layer of pink stuff. This builds up over the years, until it forms a thin black lining, some of which flakes off and is suspended in the flush water. The only way to lose it is to stop using anything in the flush water and gradually the bits will reduce. If you're troubled by only using plain water then get a spray bottle, fill it with diluted pink and spray the bowl after use. It is only a perfumed pink liquid after all, with no magic properties and it's use isn't mandatory.

I have been motorhoming and unable to log in to this site for some odd reason on my phone but thanks to all of you who have replied.
I am going to abandon the Pink and see if it will clear up now.
I think that the Blue chemical in the cassette is fragrant enough as we never do ''Two's in the loo''.
A big thanks to all of you for sharing your experiences with me and I hope that I can be of help to you sometime.