The Loaches will still continue to dig, a lot. I do not think any method of capping is going to solve the problem of fines coming up when the substrate is disturbed.
I see 2 approaches. Both mean a lot of work:
1) Remove the substrate, rinse it like crazy and redo the tank, perhaps buying more substrate if it turns out the material you have gets mostly rinsed away.
2) Rinse it in the tank by doing a lot of deep vacuuming.