It has an acrylic lid
You'd be surprised what fish will jump through. I lost two yashas to jumping and that was
with a lid. Plug all gaps. I'm being serious. I ended up just getting a goby with a really fat head.
If you can get one to eat, they're kind of finicky, a clown goby may work. Also, many enjoy neon gobies. A mated pair is very pretty.
A list of possible inverts that would make a fun and active little pico tank. Bear in mind, though not all are compatible together.
banded trochus
dwarf ceriths
Caribbean porcelain crabs (don't mix with the porcelain anemone crab or other true crabs)
Porcelain anemone crabs
Pompom crabs
Scarlet hermits
Emerald crab (can become a predator, Mine have been saints, but that's not typical)
Skunk cleaners (a small one, they can get big)
Sexy shrimp (fun and can be kept in groups, they like eating though)
Blood shrimp (reclusive)
Electric blue hermits (big and clumsy, but fun, can prey on snails)
Nassarius snails.
The list goes on, but a tank like that provide a lot of fun activity. Barring my one panda goby, my 5.5g nps tank is essentially an invert only tank.
Up to you.
Do some reading and don't hesitate to ask us questions.