What kind of snail is this--and how did he get here?

Hi all, I'm new to this website, so I'm sorry if I make any posting etiquette mistakes or anything like that. But essentially, I don't know much about snails and I'm pretty baffled as to how this tiny guy got in my tank.

I got three Nerite snails about 3 months ago to keep my betta company. The tank started to get pretty foggy and despite my half hearted water changes, it remained that way for maybe a week and a half. One evening, I noticed this tiny snail. I'm assuming it's a baby due to its size? This is a freshwater tank, but I had been adding aquarium salt (somewhat irresponsibly.) I initially assumed I had accidentally made the water brackish, allowing my Nerites to breed, but now I'm not so sure. I've also noticed the baby sticking close to one Nerite in particular, and I have no idea why. I have not noticed any Nerite eggs or anything like that in the tank.

Someone suggested this is a pest snail, but I haven't had any live plants in months. I've had the marimos for about a year, but other than that, my tank is pretty devoid of organic plant matter since I'm terrible at taking care of them. I don't think I've had any plants or wood or anything for about 2, maybe 2 1/2 months.

What kind of snail is this? It is a baby, right? What should I do to take care of it? I did a tank flush to clear the up the tank, and now this snail is pretty lethargic. Is it in danger from my betta? I don't know much about snails, so any information would be greatly appreciated. I included some pictures I've taken over the last few days, and I feel like he looks like a pest snail, but I would love second opinions.

It is one of those snails called by various names, pond, tadpole, bladder, pest etc. I have those in my tanks. Look at the first two snails described in this thread http://www.fishforums.net/threads/freshwater-snail-species-in-the-hobby.424364/ yours is one of the first two.

it could have come in as an egg on the plants or wood you got a couple of months ago - but in that case there may be more of them.

Essjay has answered the snail question. I will pick up on something else, namely salt. You do not want to be adding salt to a tank containing any freshwater fish species. A betta was mentioned, this fish does not appreciate salt and it is harmful.

Welcome to TFF.

Byron.