You've had a result there; no algae build-up in a black tank.
They supply them cleaned according to their website, so all I'd do is fill with pure, leave overnight, stir with a clean long stick/cane and check TDS. If it's lower than about 5, use the water. If not, let it out of the valve at the bottom and refill. The tap at the bottom is HARD to move. And the flow will be fast and flooding, so make sure it's not going to cause damage.
To see how full it is, just tap the side and you'll quickly work out how much is in there as it'll either resist your push (below water) or not (above).
Top tip: If you need to drill any holes in the top of the IBC, put a cup inside under where you're drilling so the swarf doesn't drop into the tank.
Vin