Makes sense. If memory serves me correctly there was an RC post/thread that listed the types of bacteria that each ingredient in VSV would grow. The vodka was probably feeding a free floating bacteria while the bio pellets had a carbon source that produced the bacteria growing on them or on any surface. Turning the UV off allowed the free floating bacteria to grow and combined with the bio pellets produced a rapid no3 drop.
If your redox had started dropping after you shut the uv off this would confirm increased bacterial load in the tank. Now you have to be careful with fish load and night time oxygen demand as bacteria consume oxygen 24/7.
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