These small volumes of water or whatever you wish to call em(looks like a duck,quacks like a duck) are not suited for the well being of fishes.
By what means did you determine this? What are your criteria for evaluating the health and happiness of fish? What is the minimum amount of water required to eliminate signs of distress?
Let me clarify that I believe animal welfare is very important. I do believe that fish are able to suffer, are able to feel pain, and are able to feel rudimentary emotions like fear, curiosity, relief, and happiness (if by happiness we mean emotional pleasure, not perpetual bliss, which is impossible even for humans). I have read books about animal psychology that make a good case that fish can feel frustration. They can learn and remember. But the problem with creating standards of welfare without evidence is that there's no reason for anyone who disagrees with your standard to follow it. They could just as easily say yours is too low or too high compared with their equally untested standard. Which is what I am doing.
Maybe 5 out of a 100 hobbyist's could sustain a few small fish in these small container's successfully over the long haul.
A quick Google search states that roughly 12,500,000 American households keep fish as pets. Let's assume your definition of "hobbyist" is super tight and applies to only 1% of them (125,000). In that case, roughly 6,250 households are skilled enough to maintain fish long-term in a nano tank. I don't think it's unrealistic to expect that the OP might become one of them.
These 5 people don't come looking for advice on how to successfully accomplish it.
Sure they do. Knowledge can only come from two sources: personal experience, and the verbal or written records of others' personal experience. If we limited ourselves to the first, we would all be keeping handcaught minnows in buckets. The OP is doing exactly what you should when you're about to try something new: ask others for advice. And you're using that as evidence against him?
Is widely known among experienced hobbyist's, that these small container's are much more difficult to maintain healthy/stable condition's in ,suitable for long term health of life therein.
Considering this guy has a healthy stand of plants growing in the jar, I think we could say he's experienced. He probably has a working understanding of water chemistry and the need for stability. That's a good enough foundation to start branching into more difficult areas.
May as well give it those that don't know straight.
Yes, let's discourage people from trying hard things. How do you think the magical 5% of aquarists learned to keep small tanks? Trial and error. Why should they be allowed to do it and not the OP?
The OP is asking for advice so as to achieve the same result as more experienced aquarists with fewer mistakes. Instead of telling him how to avoid the pitfalls, you're saying it can't be done and he shouldn't try, whereas he can clearly see that it can be done (since others have done it), and you are preventing him from learning how.
The fact that some video exists, showing a group of small fishes swimming about in these small container's, does not mean they survived more than a few day's or week's.
I could post up a video of an Oscar in a ten gal tank , or a dozen Koi in a 55 gal,
In reality the two examples would not help anyone(fishes either) other than those who search out someone,anyone, on these forums also doing the wrong thing to make themselves feel better about doing likewise.
I agree, a few minutes of video says nothing about the long term viability of any setup. But it's not hard to find entire breeding setups with tiny barren tanks packed full of fish. Killifish breeders in particular like to use very small tanks. In laboratories, zebra danios are bred and raised in tanks less than 5g, often without any decor, as you can see here:
How do these tiny tanks work? The water is kept clean, the temperature and pH are kept stable, and the fish are observed daily for signs of disease. If the OP maintains those conditions in the jar, the fish should be fine.
Oscars and koi are unsuited to such small tanks because they both grow to very large sizes. Would you agree that a 10g is enough for a betta? An Oscar is over 5 times the length of a betta, and easily 20 times the weight. Should their tank be either five or twenty times larger than a betta's? H Formosas are less than half the length and a quarter the size of bettas, so why should a 2.5g jar be too small for them?
I recall a young girl on another forum who prolly had thirty small bowls in her home she shared with her mother that each held a Betta or perhaps as many as 6 females to a bowl.
She was indeed able to care for these fishes...
In other words, it can be done.
...until an injury left her temporarily unable to. And as she recuperated in the hospital,she left the small bowls in the charge of her mother.
Long story short, when she was able to leave the hospital and return home,she was saddened by the death of a few fish and overall condition's of these small death traps.
She then proceeded to publicly flame her mother for not providing the same care that she provided.
It was unreasonable to expect that her mother or anyone else could or would.
I agree it's unreasonable to expect others to be able to step into your shoes when you have a hobby that requires a lot of expertise and commitment. I think it's equally unreasonable to ban everyone from diving that deeply into their hobby. Just about every high-tech reefer would have to quit.
Just as unreasonable in my opinion, to expect much in a small jar where water chemistry /stability are much more quickly influenced than in a larger volume of water.
I don't think the OP is expecting too much. He didn't ask how many goldfish he could put in the jar, he asked what species of fish are small enough to live comfortably in it. He doesn't expect a large fish to live in it, he expects a small fish to live in it. Which is possible, though difficult.
Your objection seems to hinge, not on the practicality of maintaining fish in the jar, but on the morality of keeping fish in small spaces. Centuries ago, keping fish in small pots and bowls is how the hobby started. Most people who get into the hobby start with a small bowl or tank. You can't deny that fish can live in very small containers. You can't even deny that they can breed and several generations can live out their natural lifespan in such containers. So, on what evidence, aside from your own personal feelings, do you say that these small containers are completely unsuitable for keeping even the smallest and hardiest fish?
Exactly. Why should your opinion be given more weight than mine or the OP's? Give me evidence supporting your opinion, and mine may change.