sylvestris
Ya, if the water was too warm that would damage the roots, then when it cools down the plant tries to generate new roots. But 17 degrees C is not too warm, in fact 20 degrees C (68 F) is better. Cold water wont cause the roots to brown, but it does cause stunted growth. The optimum water temp is between 20 C and 22 C. If the
nutrient solution is only 10 days old and not cloudy, and you say you don't feel anything slimy (slippery), and the plants look otherwise healthy, I only have 2 likely suspects. First I forgot to ask if the nutrient solution has a bad smell, but if it isn't cloudy I don't think it would.
So I would suspect that it's probably due to either tinting from the
nutrients themselves (especially if they are mixed on the strong side), and/or the submerged roots may not be getting enough air bubbles, and may be suffocating. But as long as the plants are looking fine and you are happy with the air bubbles, I wouldn't worry about anything at this point. Although I would raise the nutrient temp to 20-22 C.