Hey annesgarden~
Lotus are actually a lot tougher then you would think.
If you decide to divide, now is the perfect time to do so. I have lotus
growing in 30 gallon pots with just dirt and water and I have some in inground bogs with other plants.
The black gunk in the bottom of the pond is actually a combination of rotting plant materials so it is nutrient rich. The only thing I can say about if you should or shouldn't tackle this job is every year it will just become a bigger job. Lotus are fierce growers and can take over a small pond in 2 seasons once they escape the pot.
Hope this helps
Donna