Hi Guy!

You are right you can filter a pond with just plants
as that is what I do with my frog bog.(800 gallons
and stuffed with plants.)
The one thing you want to watch out for is getting too
many plants and too many inhabitants in there at
the same time. Plants start using oxygen in the night
hours and the animals can be in severe distress by
morning. I had this happen in my bog when I kept
fish in there. Just before the sun came up they were
gasping at the surface. The frog bog is now fishless.
(I use Mosquito Dunks to keep those critters out.)
A pump will keep oxygen in the water overnight.
So it will probably depend on how many tadpoles you
end up with.
Basically our ponds aren't like Mother Nature's ponds
as ours are smaller and more heavily stocked which is
why ponds, especially those will fish, need filtering and
water movement throughout the day. So it is a question
of balance.

kathy