For example:  
     "Where's the fry?"

     "Well if this snail would move a little, and push that one out of the way, and they make a little peep hole, you will be able to, maybe, possibly, see one." ;o)

I've used a 2.5 for my fry before, and the light to heat the water does cause a lot of algae growth.

I used a $1.09 Dobie, or other nylon/sponge combo to clean mine.

A not so quick perusal of several sources from google gives an unclear answer on snails carrying disease. In some cases that would clearly be the case. I'm not sure it would be for Camallanus if the snail was treated.

If it was a pond snail, I'd crush it quickly so dying would be brief. The risk wouldn't be worth it. Please tell me you haven't named it yet. ;)

A lot of new "plecos" are carriers of disease by the way. They also need a quarantine. The only small one for a 2.5 gallon tank I would recommend would be Otocinclus (or Parotocinclus and near relatives). However they don't take chilling well and small tanks are vulnerable to that.

Young bristlenoses (Ancistris) could be shoe-horned into such a tank, but if you didn't feed them veggie flakes and tablets they'd starve anyway. Water changes would have to be frequent.

For what the stores want for a 2.5 gallon tank, you could buy one, maybe two 10 gallon tanks on sale. :)