[Ddavis]

I have recently plated some hornwort stems in my tank and I see that they grew really tall really quick.
The number gap between the the leave nodes is around 5-6cm, which is really diffrent from what I see in pictures on the internet.
The tips also look yellowish brown and not the dark lush green that I expected

 

[Michael]

Welcome to APC!

Unfortunately, the plant in your photos is not hornwort, but Cabomba aquatica. It is a more difficult species to grow, requiring fairly high light. Your plants are streching to reach to reach the light at the surface.

 

[Steven F]

Many LED lights don't have a lot of red light but instead a mostly blue white light. If there isn't enough red and too much blue the plant will grow taller. I have see a plant that normally is about 2" tall grow to 4 to 5 inches. I added more red light and it now stays about 2".

In your second photo the older leaves look fine but the newer ones didn't look healthy This typically means a macro nutrient issue. Perhaps not enough nitrogen or maybe a phosphate deficiency or a .low GH (calcium magnesium) level.