I'm not great at identifying fish other than those with particularly distinguishing/distinctive features or those I'm familiar with as potential occupants of my own tank.
My initial thought was a filament/blackspot barb with this picture https://www.fishbase.se/summary/Puntius-filamentosus looking the most similar, but, the more I look at other pictures of filament barbs and the picture of your own lovely fish again, the less confident I am. Is / has there been any colouring of the fins of your fish?
I’ve attached a picture of a fish that I acquired about 6 years ago. He was the only one of his kind and Is chilled out, non aggressive fish. He’s about 5 inches long, 2 inches tall and of thick set. He was obviously smaller when I got him but still quite a Good size.
Does anyone have any suggestions as to what it could be?
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.
Do you happen to have a large spare fishtank / large container (e.g. a 145-litre Really Useful Box) plus filter and/or airstone that you could move it into? It might be that it is less vulnerable in there and can be treated more easily. Does it look injured or bloated in any way?
If injured (perhaps by a bird), then it might be that it requires anti-finrot treatment. If bloated, it might require fasting or being fed peas (with shell removed) to help reduce the bloating until it's well again.
With all due respect, Suzanne, my response to you yesterday late afternoon / early evening containing the relevant links was not because I "couldn't be bothered to type anything" - I was not capable of doing so at the time (and for very valid reasons I should not have to and indeed won't go into).
The only sarcasm I have seen on the forum in connection with your posts is your rather rude response to that set of links - which were provided to enhance your understanding around the issues - as well as your quite disrespectful post here.
Those who reply on this forum do so in an entirely voluntary capacity - each of us have other aspects going on in our lives, and we reply when we are able to. There are fewer users of forums nowadays, so the pool of respondents is lower currently, but that may change in time. [Fishlady's responses helpfully and fully answer your queries, though - I have read them but wouldn't have anything to add to them, and hence why I and likely any other forum users haven't, to avoid duplication.]
Re: Advice on cleaning chemicals and fish tank.
The second link in particular details how to do water changes, the equipment required, and the importance of having no chemical traces (such as the example you give about washing-up liquid) - the sink itself would not be needed, just the water from the tap plus the fishkeeping bucket. That and some of the other links also refer to how to clean items from the tank and that nothing else is required besides water.