sounds to me like a bacterial infection. i had the same problem before. Dr. Cary said work it up to 2 tb. spoons salt/10 gallons for 10 days. 50% water changes daily adding the same amount of salt to the new water during the change.
for young discus they like 88 degrees. if they don't eat at that temp, raise it to 92 until they start eating again.
your tetras probably won't like the heat though, most of us keep cardinals if anything. they withstand the heat much better.
PS: good job providing proper info