Unfortunately running the tank empty does nothing to cycle it and the quick start bacteria will have died as there is no ammonia source to feed them. Your water will test fine at the moment because it's just a tank of tap water, but if you add fish ammonia and nitrite will build up and harm the fish. Please read the article I linked to above - you need to cycle the tank with ammonia to simulate fish waste.
Oh OK. I have plants. I thought it says plants can help create bacteria? How do some people make filterless tanks on youtube and their fish survive? They say they often do water changes. Can't you just do more water changes at first to save the ammonia building up too much, while bacteria becomes more established in the filter?
Plants don't create bacteria. They can use some ammonia and nitrite, but filterless tanks only work if you have a huge number of established plants and very few fish - think so many plants that you can barely see the couple of fish in the tank.
YouTube is not a great source for information on fishkeeping, to be honest, there's a lot of dodgy information on there and in this case, you wouldn't be able to tell if those fish were being burned by ammonia or if they even survived beyond the end of filming.
Traditional fish in cycles work by starting with 1 fish and doing daily water changes then after a week or two adding one more and so on, but the fish do suffer ammonia for the entire time it takes to cycle and it's an awful lot of work changing around 50% of the water daily for 6-8 weeks. A fishless cycle is less work, more reliable, allows a full stocking at the end and most importantly saves the fish from unnecessary suffering and/or death.
Following up on this thread re different species of fish, there are water hardness ranges acceptable to each species. Fish kept outside those ranges might survive for a while but won't thrive, and the inappropriate water results in osmotic stress, kidney damage, affects their immune system and in turn results in premature disease/death. Of the fish suggested, it's best to have fish near the middle of the hardness range - your 20 German degrees hardness or 355 ppm would be great for platies (maculatus or variatus) or endlers [and ok for celebes rainbowfish* and some other lovely fish species albeit not coloured]. However, it's on the very cusp / upper end of the acceptable range for the other suggestions (x-ray, red eye* or false penguin* tetras; white cloud mountain minnows) or the more colourful cherry barbs. It's too hard for the upper end of 215 ppm for the neons or cardinals. Hope that clarification is helpful.
It sounded like you were happy with the option of platies which is great as they are ideal for your water, and endlers are another option. I didn't/don't think you're open to the suggestion of less colourful fish but they do have alternative merits - patterning (eg tiger barbs*), fin or body-part colouring (eg x-ray tetras, red-eye tetras, black widow tetra*) or behaviour and especially if in large groups (eg false penguin tetra, danio/microrasbora erythromicron). Initially, I was attracted to guppies and platies but then realised they weren't suitable for my very soft water; it took me a few weeks to come to terms with this, then I opted for x-ray tetras because of their fin colouring - which I now appreciate all the more in contrast to the cardinal tetras' body colour but comparatively bland/colourless fins. [* need a tank of minimum 70L, 106L, 70L, 80x30cm footprint, 70L respectively; think most of the others are fine with minimum 60x30cm/54L]
Thank you for the information. Yes I'm starting to think Platies are the most realistic option and liked how they look in the store. They seemed to have a personality. To make things more picky, a reason I'm not sure about minnows, is because they seem to stay near the middle and top of the tank, and platies seemed to vary where they swim, sometimes going down to the bottom. Just that I've decorated the tank floor, and would like a fish that might appreciate it. I'm looking up the others you mentioned out of interest, and like the red eye tetras. X-ray are interesting. Reading up on endlers. Didn't know about endlers before. They're pretty and so are rainbow fish. I remember liking lamp eye fish in the store. It says online they come from Africa, and not south America like Cardinal and neon tetras. Are lamp eye not tetras? What's your opinion of lamp eye for hard water?
On another note, it says cichlids are suited to hard water. I think as they're large and have bigger eyes, it felt as if they were looking at me, and I felt a connection to them in the store, but I'm guessing you need to be a bit of an expert in that type? Also all the tanks for them seem to be made of bare rocks, and my tank is quite green.
Lampeyes aren't tetras, they're a species of killifish from Africa. Unfortunately, they aren't suitable for water as hard as yours.
Only certain cichlids are suitable for hard water; those that originate from the Rift lakes in Africa (Lakes Malawi, Tanganyika and Victoria). Most likely you've seen Malawi Mbuna cichlids which need a minimum of a 200-litre tank and a huge pile of rocks to create territories and hiding spots as they are aggressive fish.