There are a couple of reasons your water might turn brown while using Nualgi.
The first is adding too much Nualgi to your pond. Overdosing will create a diatom over-bloom which will discolor the water to a light brown.
Another possible reason is algae cell die-off combined with poor filtration. To determine which condition is causing your water to turn brown, fill a glass container with water to a minimum depth of 12 inches. Let the glass sit undisturbed for several hours, then examine the bottom of the glass. If the bottom is littered with debris, your problem is caused by a filtration issue. If you don’t see debris at the bottom, the brown water is caused by a diatom over-bloom. This can be easily addressed by holding off on your next dose of Nualgi till the brown tinge is gone.
Remember: a little Nualgi goes a long way!