I bought 3 kuhli loaches yesterday and without thinking about the substrate they can go in. I have eco complete black in one tank and eco complete red in another. The black seems more rough then the red one does. Should I return them or can I put them in the red eco complete.
Get some sand. You can use play sand, just put it in a bucket and run a hose through it, until the water runs through clear. Stir occasionally. Most home improvement stores sell play sand pretty cheap. That will be the best option.
I think they will technically survive but not be very happy. I had 4 survive a year on a 50/50 mix of ecocomplete black and aquarium sand. They weren't able to dig in it and I think that may have contributed to them hiding all the time because they didn't feel secure.
Black diamond blasting sand is cheap at Tractor Supply. PetSmart and Petco both sell black aquarium sand. If you want tan sand to top your red gravel, look for pool filter sand at any big box hardware store. I prefer pool filter over play sand because it's easier to clean and clouds your water less.