It would be good to know if you have soft or hard water if you go on your water companies website you might be able to find some info - do you ever have problems with limescale?
In a big tank like this with neons if you have soft water I’d do a South American tank with some dwarf cichlids like apistos or laetacara maybe some Cupid cichlids. Some of the nice corydora species, some headstanders, red lizard whiptails, hatchet fish. You might be better with cardinal tetras too very similar to neons but get a bit bigger and also are a bit healthier as neons are a bit over bred these days.
Managed to find this on the united utilities website:
Typical water hardness: Soft
Hardness clarke: 2.73
Not sure what this means, haha!
Just having a look a Google images, apistos and red lizard whiptails look interesting!
Right, i have a daft question, this is the smallest black sand i can find on Amazon: https://www.amazon.co.uk/dp/B018K4Y9IG/?tag= I assume this is to big?