I would start with an ich medicine, my preferred one is ich-x, which in my experience does well on loaches. There are some goodness resources on how to treat tanks with ich, but make sure to act fast, ich can spread quick. I will paste below the article I use. The medicines mentioned in the article I have seen can be harsh on loaches, again, Ich-X is my favorite medicine for Ich.
How to Treat Ich in Aquariums
1. Over the period of three to five days, raise the temperature of your aquarium to about 84° or 88°F.
2. In higher temperatures, water is unable to hold high concentrations of oxygen. To prevent lack of oxygen, you can add a temporary air stone to your tank.
3. Remove any chemical media from your filters
4. Siphon the substrate in your aquarium while draining your tank to 20 to 50 percent.
5. Fill your tank and dose one dose of a Formalin and Malachite Green based medication according to the directions on the package.
6. Repeat steps 3 and 4 daily until Ich is no longer visible on the fish.
7. Dose your tank one last time for precaution followed by a 50% water change.
8. Slowly decrease the temperature of your tank until you have reached your desired temperature
9. Monitor your fish closely looking for any signs of Ich.
