Neon tetra

Pros

• Once you get your water sorted out I would return or rehome either the betta or the widow tetras, and add more neon tetras (and widow tetras if you kept them) to keep them happy and healthy :)
• If you want that tight schooling fish look cherry barbs, neon or cardinal tetras and harlequin rasboras are amazing.
• Neons are fine
• after consistently failing to keep bettas alive for more than a year and a bit I tried out some tetras in my 10 gal and they seem to be doing amazingly, I’ve had no deaths and they’re all vibrant
• Would a couple chili rasmoras or neon tetras do okay?

Cons

• 3rd neon tetras can be pretty fragile, being sensitive to poor water quality and often victim to poor breeding, so be prepared to lose some or all of them.
• 10 Neon Tetras (or Green Neons)
• Neons are more of a shoaling fish than a schooling species, unless there's a predator, or if you have 100 in a 75-gallon tank.
• Neon Tetras have been in tap water for many generations, and can easily tolerate water from 4-5 pH like their ancestors as well as a pH as high as 8. Neon Tetras are fragile in a different way, though - they're really sensitive to nitrog...
• I know Neons seem to be a super easy fish to keep but they actually require fairly large groups to feel safe and a lot of space to swim.