Oak leaves are absolutely fine. They provide tannins similar to those provided by Indian Almond Leaves (IAL), make the water slightly more acidic, and have mild antimicrobial effect.
What water conditioner are you using? Since your tap water has a small amount of ammonia, you may want consider the following options:
- Use a water conditioner that temporarily neutralizes ammonia by converting it to ammonium. The conversion only last for about 48 hours though. So you may need to do more frequent water changes than the usual recommendation.
- You could use some bottled drinking water in lieu of tap water. Make sure it's drinking water, preferably "with minerals added for flavor." The minerals are electrolytes and are required to maintain good health. (Do not use distilled water!)
Increase the humidity in the tank. This will help him breathe more easily, and has a soothing effect. You can put plastic food wrap over the top of the tank to do this. Be sure to leave at least several inches of air.
If possible, find some mosquito larvae outside. Rinse them with a little water before putting them in the tank.