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From many aquarists' experience, eco complete under the sand should work.
According to my research long time ago, kuhli loaches tend to be happier in sand substrate. I used to have fine gravel, then I changed to sand. They like excavating sand, which they cannot do with gravel or large piece substrate.
If you are worry about plants, I will say that plants are just fine with sand.
My kuhli's have never tried to dig in the sand. Or any other substrate I kept them on. But maybe that's due to me having lots of places for them to get out of site and under if they wanted to hide.