Goldfish - Sudden Fin/Tail Damage

Question
Hi Nick.  I hope you can help me.  I have two goldfish (a comet and an orange
ryukin) in a 40 gallon tank.  I have had these same two fish for almost 4 years
(upgrading tank size when necessary).  My question is that for the last 4
years I have not had the problem that I have now.  The orange ryukin started
to show black edging to his fins and tail about 3 weeks ago.  At first, I
thought it might me a color change as it looked almost decorative instead of
damaging.  Then it developed this black spot on its tail with a slight bubble to
it (last week) and the bubble is now gone but the comet started showing fin
and tail damage as of this week.  The comets tail looks frayed with whitish
edging to her all white tail.

I have made no sudden changed that I can explain this problem on.  When I
tested my water last week everything was fine except that the pH was on the
lower side.  I added some pH up 7.5 powder and some aquarium salt (very
little) for the wounded fins and tails.

Today I bought a different kind of test kit which is the Mardel 5-in-1 strips (I
have previously used the dropper kind of test kits) but this Mardel test comes
up with some scary results.  Nitrate is at 80ppm, Nitrite is ok at 0, Hardiness
is 120ppm, Alkalinity is 80ppm and pH is about 6.8.  Ammonia was done with
the old dropper test and that shows to be 0.

Could the fin damage be as a result of the higher Nitrates, the Alkalinity and
low pH?  Why would the old dropper tests come out fine (meaning ok Nitrate
level) and the new strip tests show a high Nitrate?  I never tested for
Alkalinity and Hardiness before and now the more I read about how Alkalinity
and pH correspond I am wondering if that should be my next step.   I am
considering putting baking soda in to help with the low Alkalinity.  

I've had these fish for 4 years and i'm puzzled as to what is going wrong.  
They both have been beautiful fish with long, amazing tails and fins with
never any problems.  I feed them high quality foods and supplement with
frozen veggies.  Please help me figure out what is wrong.

Answer
Hello Monika!

Yes, it is definitely Nitrate burn. Water changes should be 25% twice weekly minimum.

Drop tests are much more accurate than strips, however they expire quickly, which is likely why they were so inaccurate. I would trust a strips results over an old dropper.

-Nick