I am sorry to disagree, maybe i`m wrong, but i think that these is a lymnaea (bad snail), not a physa (good snail)
Those kind of snails usually comes with shop plants here, and 90% times are physa.
Physa is a good gardener and never eats plants ... but i dont like them because breeds too fast, and tank goes full of them in few time
(i like planorbis a lot more than physa to eat algae)
Lymnaea its like a physa, and also breeds like a physa, but its a bad snail because it eats plants.
The difference is in the shell`s turning direction (mmm i hate my english sorry)
Lymnaea shell turns like a clock
Physa shield turns in the other way
I put a link where it talks about it ... in spanish ... sorry me not found it in english , but you can see the photos
http://www.drpez.net/panel/showthread.php?t=145461
We are very far, so maybe i`m wrong and these is a NZ snail ... maybe another photo ... but i should bet for lymnaea. What do you think?
Very nice fishtank