Would merpeople be able to survive in relatively still water?

Question

So merfolk are obviously fantasy creatures, and their biology depends entirely on the designs of the writer. For my merfolk, I am leaning less into the accuracy territory, and simply looking for plausibility, correlation with real life science.

In my story, merfolk are captured by humans and put into "scale farms" where they are subjected to the barbaric practice of having their colorful scales sheared off to make gaudy jewelry and such products. The scales do grow back, but it is a painful humiliating experience nonetheless. Just as cruel is the environment that the merfolk are in. Trapped in a small cove closed off by a dam, they are confined to a small space with little room to move - and there is not a great deal of water flow in and out, so the water is lacking a great deal of oxygen, leading to the merfolk feeling very low in energy.

This is my theory, at least. In trying to find information on this the result seems to be that real life fish do suffer from being in still water, but it depends on the species -

Further, that is in artificial tanks. While this pool would not have the movement of the nearby ocean, it would still have wind hitting its surface, and the merfolk could try to peek above the surface for a moment to get a breath (they can breathe air in my story) as long as they aren't caught.

So yeah, this is the groundwork of my general question:

Can the merfolk survive in this environment? Further, if they can, would this even cause the effects I'm envisioning?

As I said at the start, I am asking for plausibility here. Obviously merfolk do not exist. The question is more - using your knowledge of how real life aquatic species (and humans) breathe, do you think this sounds plausible?

Further information - the humans are at a quasi-1850 level of technology, though this farm is a smaller one, lacking in the equipment for specialized pumps for circulating water. They do have locks to control the water flow in and out. I'm trying to decide if the humans do know about this oxygen in water thing, and they will let water flow in and out to keep the merfolk alive, or if they only do it to adjust the water level in the farm.

Answer 1

Based on my childhood experience with goldfishes kept in a bowl, not replacing the water would quickly result in a dead fish.

Unless there are enough plants and algae in the water which can photosynthesize some oxygen.

At the end don't forget that all the plants you find in those sealed, self-sustaining ecosphere glass balls, can only keep a few little shrimps alive. You would need quite some plants to keep alive a large population of merfolk.

Answer 2

Yes and no

put a lot of humans in a not properly ventilated room and already after a few hours they will feel drowsy or get headaches from reduced oxygen, how. Put a few elderly or otherwise not perfectly healthy in there and soon you will see them collapse followed by death if nobody helps. Also I guess the descaling leaves at least some wounds. Under these circumstances those will be more likely to infect instead of heal. Especially if you consider that the merfolk also defecate into the still water, polluting it with every passing meal.

I guess most merfolk would survive quite some time (and suffer miserably while doing so) since the human body can take a lot of strain. But I doubt under these conditions that their scales would heal in a reasonable time or at all. I don't think you would be able to scale most of them more than once or twice. Given that merfolk will likely not reproduce in this environment (like most fish), a bit more species-appropriate keeping with flowing water and a bit space to move and the option to come to the surface to breath would increase the output of the farm while also reducing the need to capture new merfolk frequently.

However if capturing merfolk in big numbers is easy, even better than a farm would be to just capture them, scale them and release them again.