The Red Onion snail (Neritina sp.) is a larger species of nerite snail. "Red Onion" nerites have shells ranging from orange, to brown, to bright red, with black spots travelling in a line down the spiral.
These fully-aquatic snails range from .5" to 1" in length, and a single one can live in filtered aquariums as small as 3 gallons. Snails generally have a high bioload for their size; be careful not to overstock as ammonia will build up quickly (especially in small aquariums). These snails do best in water temperatures in the mid-60's through the mid-80's, and prefer more alkaline water (a pH between 8.1 and 8.4 is ideal, but they will do well in neutral water). Their lifespan is 1-2 years on average.
It is important to have a tight-fitting lid if considering nerite snails. They are very good climbers, and will occasionally climb out of the water. It is important to provide an inch of space between your lid and the water's surface to allow the nerite snail to leave the water periodically.
These snails are extremely effective algae eaters and do best on a varied plant-based diet. If you do not have enough algae in your aquarium, supplement their diets with algae wafers, veggie flakes and pellets, and fresh or blanched leafy greens such as kale or spinach. It's very important that your snail is consuming enough calcium for their shell health, either through calcium-rich vegetables or through additives such as liquid calcium or cuttlebone.
Nerite snails are non-aggressive and can be kept with any docile species of aquatic animal. Don't keep them with any fish large enough to swallow them, or animals prone to eating snails like some species of loach, pufferfish, or assassin snails.