Kuhli loaches are dying

Not sure what’s going on. Tank was set up Jan 4 and water parameters have been stable for about a month, maybe more. Ammonia/nitrite 0, nitrate 10-20. Water temp fluctuates a little between 77-79. Pretty heavily planted tank with several rhizome plants and there’s a sand section of the tank as well. One of them died yesterday and it looks like another is on its way out. Seems to be losing his color. The last image has a picture of one that looks healthy