Many factors play a role in HLLE.
Water quality, particularly high nitrates, often plays a role. Are your readings, including nitrates okay? Is your tank overstocked?
Diet often plays a role. What are you feeding your YT? Using a well balanced diet will help. Soaking the food in a particular supplement, Zoecon, will often help.
Do you have any stray voltage? A grounding probe is thought to help.
There are other proposed treatments and causes. However, the most commonly believed causes are mentioned above. Hopefully you can get this disease in to remission which will prevent it from worsening.