Since you acquired the fish along with a tank, you don't know how old it is or how it was treated by the previous owner. We established that he didn't know what he was doing when you first got the tank, considering the fish he had in there. It is quite possible that the loach (and in fact all the other fish in that tank) had been subjected to some awful water conditions with the previous owner. Clown loaches can be sensitive fish. Does it show any signs of whitespot? Clown loaches are very prone to this.
Other than that, I don't know what to suggest. The symptoms sound a bit like a swim bladder infection, but.....
I read somewhere once that loaches don't have swim bladders. That's why they have to use their fins constantly when they are above the bottom. They can't hover like other fish; if they stop actively swimming they sink. I've been googling but can only find an old article on practical fishkeeping's website.
Loaches are also sensitive to medication as they are scaleless fish; some meds can't be used at alll, while others must be used at half dose. Things to try before medication include to check it's not overeating, and that it's not constipated (full intestines can press on the swim bladder if does have one). That is where feeding mashed cooked peas, followed by at least a day with no food can help. If those don't work, the only thing I can suggest is an anti-internal bacteria medication (not an external bacteria med) but those don't always work.
Just one thing to note - clown loaches have a well earned reputation for convincing their owners they are dead. They lie upside down on the bottom of the tank, they swim upside down. That is normal behaviour for a clown loach. But they will also do this when they are sick, not just when they are being annoying and well.