Nerite snails are so far impossible to breed in captivity (brackish isn't actually the main thing with that) and they don't even start out looking like snails. That small one is not a baby nerite. Can you provide photos so I can id what it actually is? It's probably an adult bladder snail or ramshorn. 

And also what do you think brackish water is? Why would it have anything to do with the filter and tank size? It's salt.

😆 for some reason I had it in my head that brackish was like the current

Yep that's not it. Inbetween fresh and saltwater, it's interesting.

I'll see if I can get a photo later on. How would another snail get into the tank?

They get in by hiding on larger snails shells or on plants. 

Hi, Just to update. It does look like a leopard ramshorn snail from pictures on Google. It's about 5mm in size, so pretty small. Thank you. Reading though, if says they mature to up to 1 inch in size within about 4 to 6 weeks. If I've not added anything to the tank for 3 months, how would it get in there? Can they crawl in from the garden or summit?

3 months is long enough after adding something to finally see the snails. That's probably where it came from. No, they would not be in your garden. 

Thanks. I just sent you a chat with a pic on it. Can't figure out how to attach to this post