Water parameters as Far_King mentioned will help, as your present stock is going in two different directions concerning parameters, and this impacts fish health.
Depending upon the answer, it would be best to first increase a couple of the exisiting species to provide more of what they expect, which will reduce stress and thus improve their health. The neons could use a few more (six is always touted as minimum, but that is because sometimes the absolute minimum is critical; the more there are of shoaling fish the better). But more importantly, the loach needs a group. At least five, though four can sometimes work, but five or six would be best. But here we come to the species...if this is Botia striata, then five or six would be best and you have the space. I am assuming a 190 litre tank is going to be at least 90 cm (3 feet) in length. However, if this is a larger species (common names are confusing), maybe not.
Loaches need lots of chunks of wood (preferably), containing crevices and tunnels. Each loach must find its own "home." This is a highly social group of fish, but also territorial.
Brodeur30 has raised a very good point respecting the "shark." I don't know the present tank size, but this affects aggressiveness in many species, just as numbers do in shoaling fish. Individual fish within the shark species also vary, but be prepared to move him out if trouble begins. The stress this causes to other fish is not always apparent visually.
Byron.
