Why do my bala sharks keep dying when they seem healthy?

I'm pretty sure bala sharks are just another name for silver sharks. So, the temperature should be at 25 degrees (I'm from Australia, i think its 78F?) So make sure that's ok, if you dont have a heater get one. Check the brand of food your feeding them. The dodgey supermarket or cheap foods are no good... go something like Wardley Flakes or Nutrafin flakes... the more you pay for food normally the higher quality it is. Also make sure there's enough oxygen. Add an air pump if you don't have one. And check PH and HARDNESS. You can get those test kits from your local aquarium. Also if its a new tank you have set up it might be lack of good bacteria. You can get some liquid called pura water that will fix that up. And I trust your using chlorine neutralizer and conditioner to treat your water before you put it into the tank. Also get a bigger tank and some smaller fish to help it survive longer