You've had good advice from other members, so i won't repeat any of that, or try not to, except to say that nitrates are high [I assume you shake Regent #2 for 2 minutes when testing the tank too, just in case] due to the high fish load in the tanks, and too infrequent water changes. Live plants should keep them below 20ppm easily, often below 10ppm.
To the fish. Clown Loach
attain 12 inches. Or at least, they will if they have a 5 or 6-foot tank to grow properly. A small tank is only a temporary measure, so if a larger tank is not likely within a few months, I would consider re-homing them (other aquarists, store). Same with the pleco if it is a Common Pleco
, they attain 18+ inches. Both fish are in our profiles, click the shaded names to see the info.
The nonsense about fish growing to the tank size is just that, nonsense. I am amazed that store staff say this. What actually occurs in too small a water volume is what we term stunting. The fish continues to grow but the internal organs cannot develop properly, so it is deformed. Immune system is weakened, so the fish acquires all sorts of other health issues it would otherwise be able to fend off. And they always die prematurely. This is a combo of physical space and water quality; so doing more frequent and larger water changes helps, to a point, but it is not the solution.
Salt: no. While livebearers can manage with salt, they do not need it. And soft water fish such as loaches, gourami, tetra, barbs, etc should not have it at all. There is no value to regular use of salt. But there is a detriment to many fish and plants.
The fish losses mentioned initially are likely due to cycling issues (ammonia or nitrite poisoning). An aquarium will cycle in 2-8 weeks. Using a bacterial supplement can speed it up, but the bacteria still must colonize the surfaces and this takes time. You can read more here in my article: Bacteria in the Freshwater Aquarium
so I won't repeat. Other than to say that a fish may survive the initial cycling issue (ammonia or nitrite or both) but it weakens them considerably and almost always leads to increased health issues and early demise. The nitrate issue would obviously be adding to the stress from the cycling; and the salt is adding more stress on top of that. You can see how each issue in itself may be minimal, but compounded they can be terminal.
Hope this helps explain things; don't hesitate to ask questions of us.