My wife's sister has a 15m indoor pool which is rarely used.

The water was green. I think the water is circulated, but not conditioned.

Is there some type of local fish they could dump in there and/or plants that would make it like a natural swimming pond?

The short answer is no.

Static water that is not refreshed may not be 'stagnant' in the true sense of the word, but there is a high risk of schistosomiasis, bilhariasis, carcerial dermatitis (swimmers' itch), and salmonella, typical diseases contracted from swimming, for example, in quarry water that has no natural stream or spring to replenish it.

Fish may thrive in it, buy you won't

The short answer is no.

Static water that is not refreshed may not be 'stagnant' in the true sense of the word, but there is a high risk of schistosomiasis, bilhariasis, carcerial dermatitis (swimmers' itch), and salmonella, typical diseases contracted from swimming, for example, in quarry water that has no natural stream or spring to replenish it.

Fish may thrive in it, buy you won't

There is a stream going through there yard which travels through hundreds of kms of mostly farm land. Bana Nahkon Nayok. I believe they pump the water out that stream, but will have to check on it.

The water can be circulated, they have some pretty fancy looking equipment. It's a well built building and indoor pool.