That is a common mystery snail.
It is a big mystery snail.
I have two blue's that are that size or very close. If you got it from Petsmart, it's a mystery snail.
Mystery snails are a type of apple snail but not the ones you saw in those links given to you. Giant apple snails get way larger than what you have now.
If that snail you have is eating healthy plants... he is STARVING.
What do you feed it? If you say he just eats algae and left over fish food, he is starving. They need pellets or wafers to supplement their diet. Fresh kale and cucumber is good for them too to provide extra calcium.