Very Strange Nitrate and pH Readings

Question
Hi Matt,

I have a 10 gallon aquarium that has been set up for about 7-8 months now and had 2 black moors in it.  About a week ago one of the black moors appeared listless and then died.  The other fish seemed fine.  I did tests for nitrite, ammonia and nitrate.  All were at 0.  Previous to this I hadn't tested in quite some time because the tank had cycled and had been having normal nitrate readings for some time so I just assumed all was well until the one fish died.  Today the other fish's eyes were cloudy and his fins have white stuff on them.  I did tests again, no ammonia, no nitrite, no nitrate BUT the ph was at 5!  Now I guess I know why the other poor fish died.  What I'm wondering now is since the water has such high acidity does that mean all of the beneficial nitrifying bacteria would have died and will the tank need to cycle all over again?  I did a water change today and have read that I should change the water slowly so as not to shock the fish.  I also put some salt in the water to aid in healing this burns.  Poor little guy.  Any advice would be MUCH appreciated.

Answer
Hi Darby,
A cycled tank must show some readings of Nitrate.  Readings of Nitrate shows that a tank is maturely cycled because it has grown both sets of bacteria.  Both which break down ammonia into nitrite and then nitrite into nitrate.  The only way to get rid of nitrate is by doing water changes, and even then it is very hard to make it become 0ppm.  There is one way to maintain nitrates at 0ppm, which is by having a great deal of live plants in the aquarium, enough to soak up the nitrates.  But other than that it is very unusual that a 7-8 month old tank has no signs of any nitrogen source especially in a small 10 gallon tank with 2 goldfish, since goldfish eat and produce a lot of waste.  Are you using test strips by any chance? Test strips are very very inaccurate compared to liquid testing.  

Cloudy eyes sounds like a type of bacterial infection.  A low pH would never cause symptoms like that.  There is a medicine called Maracyn and Maracyn Plus that you would look into trying or any anti-bacterial med also should work.  Also for the low pH, you can fix that by adding crushed corals to the substrate.  Substances such as pH up and pH down and etc do not work due to buffers in the water so if you are looking to raise the pH of the water, I would use the crushed corals.  It should be sold at most local fish stores.
-Matt-