There's no such thing as "weak" or "Strong" fermentation, there just is fermentation.
Are you implying that your airlock was bubbling and slowed down? That's perfectly normal. Your airlock is a vent, a valve to release excess co2, it's not a gauge of your fermentation. More than likely the majority of fermentation has wound down, as ALWAYS happens. But that doesn't mean fermentation has stopped, just that the amount of EXCESS co2 production has slowed down.
Fermentation is not always "dynamic," just because you don't SEE anything happening, doesn't mean that any-thing's wrong,, and also doesn't mean that the yeast are still not working diligently away, doing what they've been doing for over 4,000 years.
You have a big beer, it's going to take a long time to chew through the rest of the sugars.
Just leave your beer alone. The yeast know what they need to do.
When it's been 12 days or so, since yeast pitch, take a grav reading, and another in a couple days.
But don't assume anything's wrong because things have slowed down, things ALWAYS slow down eventually.
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