water tank problem

Algae which grows in diesel tanks doesn't require light and under silicon seals in bathrooms where there is no light mould will grow quite well.
So maybe it is algae and until you eradicate it it will continue. Bicarb may shift it but not clean the innards of the tank. Unless washed over all internals but spray or moving the tank upside down etc, whatever you use won't contact all surfaces. I imagine a weak caustic soda solution would help. Pool chlorine solution perhaps.

What quality water are you filling it with? that may be causing the contamination.

you can try by putting in some water tank cleaner & fill only 3/4 fill & drive around a bit then dump it & fill, then put in "sudbury" aqua fresh this works well (its meant for Marine water tanks) aqua fresh is for fresh tasting odor free drinking water (you have changed the water filter if it has one?)

Rust? could be the answer!

I had a need to make safe for drinking water a SS tank that had been used for three years for grey water.

The tank was baffled so steam and high pressure wands were out. I didn't know the grade of SS so consulted a metal pickling business for advice about what not to do. (You have to be careful, you can ruin SS with some acids such as pool [hydrochloric] acid and IIRC, overly strong caustic soda solution. It depends on the grade of SS.)

First wash was a very hot reasonably strong caustic solution (but cannot recall the concentration, sorry). Filled the tank to the brim and left it for a few days, rolling it around the back yard two or three times a day. I did not want to dispose of that solution down the drain so did some chemical calculations and neutralised it with pool acid to make salty water.

After rinsing I gave it a hefty dose of household bleach solution (about 1 part in 20, IIRC) and rolled that around a few times a day for a week.

Then I put 5 litres of rust converter (phosphoric acid) in. Phosphoric acid is used to pickle SS if needed after welding. I did this just in case any of the SS around the welds had been compromised. Pickling would reduce the ill effects, according to the advice I got.

Then another rinse and a dose of bicarb soda solution, another rinse and then returned to service.

You can still see a layer of something on a baffle that is visible through the filler, but whatever it is I think it's well and truly neutralised. We've been using that tank for drinking water now for two years with no ill effects.

I always store our tanks full and if it's been a long while between trips, sanitise them and the plumbing with 1:100 solution of household bleach for 24 hours, then rinse and refill.

My situation is extreme compared to yours, Marcel, but maybe this will give you a few ideas.

Cheers

Sodium percarbonate is what you want.

Low toxicity, but rips organic molecules apart.

Sold as nappy sanitiser if you want the tank to smell pretty, or home brew bottle steriliser if you want to drink out of it. Pretty cheap, pour some in the tank, fill it with water, drive around for a day or two and rinse it out.

I think all grades of stainless will rust,it must be kept clean this is why cutlery rust if it get grease on it and no air ,next thing rust.

So, my next water tank shouldn't be a stainless steel one.
What would be the alternative?
Cheers,
Marcel

PS thanks for all your advice