We live in a new age.  Why live in one place? Why live on land only?  My dream home is an amphibious motor home.  With 4g connectivity, I can wake up anywhere and dive in a lake or go fishing! 

The very same technology that increases the efficiency of fishing boats also makes populations smaller.  There are ocean trawlers that scour great areas of the oceans.  The problem is with the target fish missing, other fish that may be less desirable take over, at least or a time

I don't know, but I do know YOU will be endangered if you swim with them!  ;)

I suppose they thin out plants in the garden so the remaining plants are healthier.  You can learn more about snail ecology with this page.

If the water is really hard ( that means it has a lot of calcium in it )  then the chances are better the metals won't hurt your fish.  On the other hand it pays to play it safe.  Here is an article about zinc and fish

Actually pond water  has been on/in the ground much longer so it will usually have a lot more dissolved minerals in it. Here is a link to learn more.  pond water