Not quite, but juveniles do need large and frequent water changes. I have recently been bitten by the discus bug, and now have a 120gal with a dozen 2.5+" juveniles. I do 50% waterchanges every other day, which is actually not very hard to do, if you have a python or similar setup.
To the OP, I would increase my water changes to 50%, 3 times a week at the minimum. Since you state that you only have a DIY sponge-filter, your tank could be building up with toxic nitrogenous compounds, so maybe add another sponge filter or increase those water changes.
Also that 34gal might be a tad small, once those discus reach maturity, most discus nuts agree that 10gal is the minimum per fish. Just something to think about in the near future, if you have the chance to grab something bigger.