see sea shells sitting on the sea shore
grab up some sea shells and put 'em in your filter (ps you can get shells at a crafts store, or maybe elsewhere, just make sure they're not painted or otherwise treated)
their calcium will dissolve in your water and raise the
ph well as making your water more stable to avoid wild
ph changes
try to keep water changes in the 10 to 15% range daily ... any more than this will disrupt your biological filtration as stated already, causing cloudyness and/or ammonia and nitrite spikes. if possible, draw your water change water 24 hrs ahead of time, add some simple dechlorinator (chlor-out, start-right, aquasafe) and then let some of the shells you got soak in the water for at least 24 hrs. Then do your water change.
your high nitrates may be due to other chemicals that have been used to raise the
PH or treat for sickness... or some areas have high nitrates in the water due to fertilizer run-off or other water contamination
after running the shells for a day or two, retest for nitrates, as well as ammonia and nitrites - your readings will be more accurate as your
ph stablizes and the water hardens up a bit.
once you stabilize your water, your fish's health should improve - all the sicknesses described are stress related.