You're on the right track. DE is diatomaceous earth, used primarily in DE filters, one of the 3 common types of filter and usually described as "the best", in terms of its filtering capability. But a sand filter with a little DE in it can easily do as well.

If you used 40 drops at the 10ml level your FC was 20ppm, what you need to keep it at as it dropped 0--meaning you are fighting an algae bloom.

So...check your FC 2-3 times a day, adding bleach or liquid chlorine as needed to keep FC at 20.
Brush your floor and walls once a day.
Vacuum to wasted every day.
Keep at it till the water is clear and the fc doesn't drop so dramatically.

Let us know what's happening.