and if your tank isn't cycled, it needs a 50 percent water change every day, if not twice a day!
As Steveangela pointed out earlier, discus need pristine water conditions-- and that doesn't mean 'clear' water, it means 0 ammonia, 0
nitrite and low
nitrate readings. Ammonia and nitrite are colorless, so you can't see it in your tank. The water may look clear, and yet be full of toxins.
since the fish keep dying every month, you need to get to the root of the problem, or else this will keep happening. My guess is that your tank isn't cycled, and the fish are being poisoned by ammonia and nitrite.
But if you could answer the questions we've asked, we could help, and not just guess at what's going on.
How many gallons is your tank and how long has it been set up?
Do you have a water testing kit?
What is the ammonia, nitrite and nitrAte readings?
How are you "cleaning" your tank, and how often?
What water conditioner are you using? Suggestion> using Prime will detoxify any ammonia and nitrite in the tank for 24 hours between water changes.