So my 65G planted tank has 3 very large Golden mystery snails. ive had them for almost 3 years and they are huge. (atleast in my eyes id say one is 3 1/2 x 3 1/3 Shell size... Thats the best i could attempt to do with a tape and being outside the water LOL.) Anyhow ive also had guppies forever as well... and all the sudden today i see them picking at one so i walked over to inspect a bit more and he is open walking around/sliding around... they are just nailing him and all the sudden he closes his shell.. the other ones are really close to him and no ones paying any attention to them just this one poor guy... I dont have a tank i can put him in at the moment. I just dont know whats going on... Any ideas???

I know this isnt going to be much help, but I just got mystery snails and did some reading on them. I read their life span is a year, so perhaps you took excellent care of him, but his number is up? The other fish may sense he is dying and be picking at him because of it... Like I said just a theory and hopefully someone else can either debunk me or back me up.

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"Being ignorant is not so much a Shame, as being unwilling to learn".-- Benjamin Franklin

Might try feeding your fish so they will maybe leave the snail alone.. You can also just get a clean bowl, fill it with tank water and put the snail in for a little while to see if it is OK.

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4ea 10 Gal all planted.
20 gal in for repair
Swordtails

Still learning...

LOL I like Don's answer, simple and effective.