bits and pieces of info about these critters are here and there around the forum. I've been doing some field research, studying the creatures in the wild and have returned with the following notes:
- you sing to get the tadpoles on you, the leeches are just attracted to you
- the tadpoles are in the entrance to Mithalis ( in a house in the top left and in the middle of the right area ), the whale's belly, the top left entrance to the abyss (3 in the top section and 6 in the middle), in the kelp forest (in the area below the below the field of red bulbs, behind a big red fish's cave ), in the ice veil bubbles ( on the left side below the crab boss )
- the leeches in the top left entrance to the abyss ( some on the far left and right darker areas of the map ) and the whale belly
- the tadpoles regenerate you as long as they are on you, the more you have on you the faster you regen
- the leeches slow you down, if you have too many on you then you'll be swimming almost nowhere
- you can get both of them off of you by spinning, switching to fish form ( the leeches will still pursue you in fish form though ) or leaving the map
- tadpoles are pretty fragile and can be killed by some enemy projectiles and by Li and pets. when harvesting tadpoles, turn Li off and swap out your pet
- the tadpoles' eye colour let you know what note to sing to attract them
- the best form to move with the leeches on you is beast form, but just spin to get them off
- if you have tadpoles and leeches on you, and you don't want to kill the tadpoles, switch to your pet nautilus, keep spinning, and he'll target just the leeches
- use your middle mouse button and target the leeches from a distance. motion triggers them off and you can snipe them out before getting near them
I am not sure if that is a complete list of locations for tadpoles and leeches, if anyone can think of any others then I'll revise the list

any other neat tadpoles or leech tip or trick?
spinning to get the leeches off while pet takes them out*edit: ok, so I guess they are tadpoles, newts have legs don't they
*edit: added locations from Xiagan and Mumruoy for the tadpoles. not sure if I have a complete list of areas for the leeches tho