Which snails do what, pros and cons?

Not sure as i class them as a pest, so i have not looked in to snails that much. I do know however that they will all help with some movement in your sand, but if your thinking of getting some snails, then i think its the apple snails that you want

If snails are found in almost every body of water across the globe, then why would not want them in our tanks?

You also find alot of other things in every body of water across the globe, that i would not want to keep in my tanks, (leaches spring to mind) as i said earlier i class them as a pest, so i would not want them in my tank. You only need a sudden die off of snails and suddenly you have water quality problems

my 30g tank is becoming a bit over run with cone shaped ramshorn snails. they are quite pretty in their own way and dont seem to do any harm to the tank, plants or fish in anyway. i have been thinking about getting a couple of clown loaches to deal with them but i am hesitant as the clowns could cause problems of their own, e.g. they might eat my shrimps.

although the snails do munch algae, i think, they dont do enough to stop it growing. these snails seem to spend a lot of time in and under the gravel and sand during the day and come out at night. so there is the possible benefit to the plants in the snails help keep the gravel and sand mixed up. i dont see any down sides to them being in the tank other than as clunk indicates if they all died off the decomposing snails could then cause a chemical inbalance in the tank but i dont think this is likely to happen (touch bogwood).

i also cant help thinking that the make up part of the little ecosystem happening in the tank. again this isnt necessarily a good or bad thing.

in conclusion, i dont think my snails do any harm or do a lot of benefit either and therefore it is purely personal choice as to wether one decides to allow them to stay or not.

I bought about 12 ramshorns a year or two ago...

I have since given away probably 400 or 500. So there's the only real down-side to it. They will positively overrun your tank if you don't have a snail-eater like a clown loach or something.

In large numbers, they create quite a bit or waste and in large numbers they can also do some damage to your plants.

In smaller numbers, snails are a healthy thing in my opinion..they are part of the ecosystem...and can help with eating excess food etc.

I've had Apple Snails breed, and when they're slightly larger than your thumb-nail, they make a good crushed treat.

I've had one Adult die off, and it made quite a mess in the tank. But I've (obviously) had others die off, but never noticed it, until I came across an empty shell.

All I have now are Malaysion snails. They're not much good for anything except crushing for treats. Although I have had the odd one in with my Cory Fry, to siphon off the excess feed. They are also gravel diggers.

Never have I had a problem with my plants.

CO2 injection does not go well with snails, as their shells corrode, rather quickly too.