I do tons of water changes, changing lots of water doesn't cause water quality problems in itself. It sounds like you're underfiltered &/or uncycled. You should have '0' ammonia (which you didn't test for), '0' nitrites (which you don't) and for Discus especially you should really keep your nitrates under 10 if at all possible. (I keep it under 10 for all my fish but that's just me). You should be testing with a liquid test tube kit, the test strips are worthless. You're in Boston so you should easily be able to find the kit about anywhere. Cloudy eyes are almost always due to poor water quality (as the other have stated), any ammonia, any nitrites or nitrates too high are all considered "poor water quality". The Discus may have slowly adjusted to less-than-ideal water quality and when you added the Barbs it was too much of a shock to them and they died. Just the act of adding new fish is going to stress the bio-load. Then when they die you'll end up with an ammonia spike. Sorry so long, just trying to give you things to think about.
