Cleaning would depend on the species, number, and size of tadpoles. A filter that can't harm tadpoles is a good idea; but will take around a month to cycle it's ammonia and nitrite removing capabilities. Seachem's stability can reduce that in half.

In my opinion tadpoles are not as delicate as fish fry so they tolerate small water changes. Can use aquarium airline tubing and gravity to siphon off water and dirt from bottom; then replace with same temperature and pH dechlorinated water. How many tadpoles do you have on the 20L and what is their size? Thank you !