Treat with table salt (minus the ones containing yellow prussiate). I use 3 teaspoons per gallon dosage which is a 0.3% solution. Since your tank is 120g, you may need 360 teaspoons. What you can do is add 30 teaspoons every 3-4 hours. Be sure you dissolve the salt thoroughly and add slowly to the tank. The time interval I set for you will allow the fish time to adjust to the elevated salinity. Remember that salt also adds electrolytes and a sudden increase can damage their osmoregulatory systems wherein they cannot take in the added electrolytes anymore.
Table salt should absolutely be fine especially to catfishes and loaches contrary to popular beliefs. There is no reason why you cannot use the table salt unless it is administered recklessly. As I had emphasized earlier, all it takes is careful dissolution and addition which many people have failed to follow. The salt itself will only harm the fish when added directly to the tank without following instructions of how to administer it properly.
Avoid table salt with yellow prussiate and check the labels. Although it has not harmed some fish wherein the table salt containing yellow prussiate has been used, if your tank is exposed to sunlight, it may potentially react chemically and intoxicate your fish which is why it would be more prudent to avoid it instead.
The temperature must be increased to 86 degrees Fahrenheit. Fortunately, clown loaches are very tolerant to it but you must also maximize the oxygen levels since clown loaches thrive best in highly oxygenated waters and oxygen is depleted in elevated temperature much more easily.
Treatment must be done for 14 days or 10 days after ich seemingly disappeared to make sure the ich has been eradicated completely. Failing this, you risk another infestation of ich later on and note that ich serves as an open floodgate for secondary infections since it increases the potential risk of Aeromonas hydrophila that may cause much more issues to your fish as bacterial infections.
Lastly, always quarantine your new fish regardless of the circumstances. A week is very insufficient and can potentially introduce more pathogens that cannot be detected without microscopy involved. Flukes, ich and mycobacteriosis are just a few of the many undetected yet extremely dangerous parasites. A lot of cypriniformes (with the possible exception of koi) have a high incidence of health issues particularly to mycobacteriosis which is something you want to avoid to your current stocks.
