You need to stop buying fish for the moment. You need to get some water changes done and keep your parems. up to par. Ph is way to low. needs to be 8.3. This alone is stressfull to everything. Your salinity is low as well. This needs to be 1.024. A few higher is fine. Lower is marginal.
Stop using distilled water. This will cause algae problems and phosphates. Get rid of your phosphate sponge or whatever it is you are using. Hook up your ro/di system and make some good clean water. Mix in your salt, stir and let stand a few minutes. Then do a good water change. I would do about 10 gallons to start and then 5 gallons weekly for a month. This should get your system back up to par.
What salt are you using?