I would say a big bubble filter with a strong air pump or power head should be good. And unagi can be found from Japan to Vietnam so their water temp really varies. If yours has been doing fine without one for half a decade now, I would assume it wouldn't need one now.* Eels are pretty slow so anything with streamline bodies like tetras can do fine. But with the water temp (would be great if you could give an estimated number) you can be fairly limited. I would go with snails as a clean up crew, lots of people hate them but hey, the eel shouldn't/ probably won't eat them.
*They like 25-29 C on average, I would go with around 26 because they like the fresh water rivers and breed with warmer temps. Assuming you want to control the temp with a heater or not*
With a max size of 40 cm, I would say the ideal home is 55 g (200 liter) or more, but you can get away with a 30g (120)ish long tank with a tight lid on both.
Thats the absolute perfect name for an eel, I (and probably lots of others on this site) would love to see some pictures of Una-chan.
Extra tip: When I try to find info on animals that are usually meant as food like fugu or trout, its way easier to look at the farming requirements rather than aquarium hobby specifics.