Very low water hardness

[Zafiromar]

I've recently acquired my discus juvies and they are doing fine except for one. He ws doing fine before I added the last pair of adult discus. Now he's gone dark 3 days ago and today he didn't eat. Ammonia nitrate and nitrite all test 0 but the hardness is also 0. I've added stresszyme today. The last water change was sunday. I don't usually change it more than once a week except when I get new fish. What is affecting this little guy. Should I add aquarium salt? Magnesium sulfate? Baking soda?

[Skip]

oh my.. you added new fish str8 to an existing tank?! that is probably due to some kind of nasty, the new fish brought in or he is being bullied and is lowest in pecking order.. also you need need to do DAILY WATER CHANGES! min 50 percent.. fresh clean, daily water.. is the best cure.. you need to try that then come back.. it may take 3-4 days before you see improvement..

next time dont add fish without QT, or mix juvies with adults *its a little stressful* on the little ones to see giants running around