[Mike Halliday]

I have a stainless steel tank with a large heater coil fitted, the inside of the tank & coil is coated in scale from where tap water has beed heated up.

I can't clean it out, if I start filling it with RO water will this water become massively contaminate by the scale or will it become mildly contaminated while it  slowly dissolves  the impurities.

Thanks for your advice 

[PoleKing]

Why don't you run some vikal through it on continuous flow for half an hour?
Then flush with pure for a bit.

[Mike Halliday]

Would that descaler the full tank ? How much would need to use on a 60gl tank?. Also I'm assuming it would'nt harm the pump.

[PoleKing]

Think you'd need a lot to do 60g.
Can't see why it'd damage a pump.
It's only metal & plastic/rubber.-same as a kettle.
I'd try it.