Tank #1 - use just tap water and keep hard water fish in this one
Tank #2 - mix tap water and RO water at least half RO, perhaps a bit more, and keep soft water fish.
But with tank #2 you would have to use the same mixture of tap and RO at every single water change. This means keeping some RO water on hand at all times in case an emergency water change were needed. And this tank would need to be fully cycled or heavily planted before any fish went in there as doing a large water change every day for a fish-in cycle would work out expensive.
ok have rethought fish again for plain tap water tank. Temperment wise which of these would work?
Asian rummy nose dH 12-25
black neon tetra dH 1-18
Emperor tetra dH up to 20
Endler dH 12-25
Flame tetra dH up to 25
glow light danio dH up to 18
Pygmy Cory dH up to 15
x-ray tetra dH up to 25
zebra danio dH up to 25
Honey gourami dH 5-20
These are according to the website of the shop I plan on buying from.
Please do not use any information given by a shop. They cannot be trusted to give accurate information.
Look at this website https://www.seriouslyfish.com/knowledge-base/ It is written by ichthyologists who know what they are talking about. Go through your list and see if that website agrees with the shop - where they differ, Seriously Fish will be correct not the shop.
Black neon tetra 1 to 12 dH
Emperor tetra 1 to 12 dH. Too big for 54 litres and too active to be kept with gouramis
Endler 15 to 35 dH
Flame tetra 1 to 12 dH
Glowlight danio 1 to 12 dH
Pygmy cory 2 to 15 dH. Needs a group of at least 10.
X-ray tetra 2 to 20 dH
Zebra danio 5 to 20 dH. Needs cooler water and a very long tank because of their swimming behaviour. Too active for gouramis.
Honey gourami 2 to 15. Needs placid tank mates which rules out a number of fish in your list.
Asian rummy nose- do you mean Hemigramus rhodostomus or one of the species that look similar? Hemigramus rhodostomus 2 to 15 dH
As you can see, your shop's hardness ranges are too high. The figures I've typed are all from Seriously Fish, and these are more accurate than the shop's figures.
Yes, you can keep soft water fish if you mix your tap water with RO water. But you can't keep endlers or any other hard water fish with them. It's either/or in the same tank, not both. Or have 2 tanks.
There are other issues with your list.
Emperor tetras are too big for a 54 litre tank (they need at least 90 cm length) and they are active fish which can't be mixed with sedate fish.
Zebra danios are temperate fish not tropical; they need to be kept at cooler temperatures. And they need a tank much bigger than their size would suggest simply because they are such fast swimmers and need a length of at leat 100cm to be able to swim properly. This swimming will also stress sedate fish.
Glowlight danios need a bigger tank than 54 litres, at least 80 cm long.
You mentioned in a previous post that you do want honey gouramis. These are sedate fish and cannot be kept with active fish or they will be so stressed they'll get sick.
Finally found the fish you mean, Sawbwa resplendens https://www.seriouslyfish.com/species/sawbwa-resplendens/ They need hardness 3 to 15 dH. Your hardness is right at the top of their range so should ideally be kept in softer water. If you use a tap/RO mix, they'd be OK. Provided you can find them - have you seen them at the shop, or found them on a shop's website? I found them on Maidenhead Aquatics' website with a hardness range that's too high.
If a 20 gallon long has a length greater than 100 cm, zebra danios could work, though 4 ft tanks are better for them
Both these fish need cooler water than tropical fish at 18 to 22 deg C.