I found out, soft to moderately soft water on a 1-5 scale, its 1.5
In that case, neons are fine. If it had been 4 or 5 on that scale, they wouldn't have been suitable. Or any other small shoaling fish from the Amazon.
For bigger fish, you can't go really big in 54 litres, maybe up to 2 or 2.5 inches, but a fish that would do well on its own or in pairs. How about a pair of
honey gouramis? The photo on here is the wild coloured form, most shops don't sell that particular colour. But they do sell the yellow or red forms. Or perhaps dwarf gourami, though they are prone to an incurable disease

If your tank is 60cm long,
apistogramma agassizzi would be suitable, a male/female pair. This is one of the hardier, more easy to find apistos.
Or perhaps a bigger shoal of neons and a shoal of one of the dwarf species of corydoras - that's C pygmaeus (pygmy cory), C habrosus (salt and pepper cory, not to be confused with the larger peppered cory) and C hastasus (don't know if that one has a common name). With cories you need sand or smooth gravel on the bottom of the tank.
Shrimps would also be fine, but a group of them. They don't add much to the bioload. Cherry shrimps are the hardiest of the small shrimps, and if you had just neons and dwarf cories they would breed. Bigger fish would eat the babies. Snail - I like nerite snails, they are good algae eaters. They are becoming more common in shops, but they also travel well by post if you need to use mail order.
If you do get shrimps, make sure you have hiding places for when they moult. Until their new skin hardens, they are very vulnerable to being eaten. Plants, dense ones being better, or ornaments with shrimp sized crevices are good hiding places.
Some ideas for you

My '54' litre tank (it doesn't actually hold that much water!) has
Sundadanios, pygmy cories, cherry shrimp and nerite snails, the orange-red with black markings variety.