Ok, so it seems my dad has an unintentional obsession with wanting fish he really shouldn't get/mix.

He has a 30 gallon tank that I basically take care of for him, and it has 7 serpae tetras in it. It is quite bare and just has a castle, a rock, and a palm tree figurine.

He saw some black mollies at the LFS the other day, and he wants some. However, I understand that it is best for mollies to have brackish water. I explained this to him and he got all mad and annoyed at me, saying that the store has them in freshwater, so they should be fine. I showed him that most of them were wobbly (had the shimmies and clamped fins) and he just rolled his eyes and sighed.

Anyway, could he get some black mollies and put them with the serpare tetras if I make the tank brackish? Would serpae tetras adjust to brackish if I do it slowly?

Mollys are naturally brackish, although tolerate low salt concentrations well, and can also be acclimated to full marine concentrations. They are not well suited long term to salt free conditions, they frequently fall victim to disease.

Serpæ are freshwater fish. They may survive with salt in the water, but will exist in osmotic stress, their kidney will work overtime to try to maintain their electrolyte balances. Overworked, the kidney will fail and the fish will die. Salt is a serious pollutant for freshwater fish.

The lfs is there to sell fish, not to make sure they live. If the fish die, then they sell more fish. Lfs are frequently the worst place to get advice.