Sudden increase in nitrates and low PH

Question
Hello,

I am a a fairly beginner aquarist.  My tank has been set up about 15 months now.  It is 36 gallons with 10 fish in it currently -- 3 cory catfish, 3 larger rainbows, 1 platy, 3 tetras.  I have a Eheim Classic canister filter which is maintained according to manufactuer instructions.  Fine filter media changed every 3 months, carbon every month, 1/3 of bio media every 6-9 months.  My maintenance schedule for this tank has been a 30% water change with a complete gravel vac every two weeks.  I would sometimes notice a minimal amount of brown algae starting to develop and would clean this off as well with each water change.  Until recently, all seemed well.  My nitrates never exceeded 20ppm and my PH was around 6.6.  

Here is my dilemma, I added Melafix to the tank  about two weeks ago because one of my larger rainbows (Bosemani) got himself stuck in an castle ornament and did some signficant scale damage.  Not wanting to lose him to infection, I added the Melafix because I had read that it was fantastic at speeding healing.  Well it worked fantastic.  He is completely recovered.  However my tank has high nitrate levels now.  Does the Melafix contribute to the organics in the tank thereby causing nitrates to increase.  I did several water changes and the Melafix is completely gone, however my nitrates are reading at around 40, and my PH has dropped to the lowest reading on my kit (6.0).  I have been doing modest daily water changes for the past few days after my normal 30% water change and still don't see a difference in the readings.  Should I be taking a different approach or continue with the modest (15%) daily changes?  How long should it take to correct this issue?  I also cleaned out my filter even though it was early for this maintenance and the hoses as well.  There was some organic buildup in the hoses, but in my opinion not signficant enough to cause such a problem in such a short amount of time.  Your expertise would be greatly appreciated.  thanks,
Michele

Answer
Hi Michele: the sudden increase in nitrates is a sub-result of the malafix... malafix killed off a significant portion of the bacteria in your tank... those bacteria would normally break down the ammonia and nitrate/nitrite chemicals in your water... thus the high reading for nitrate.  The pH is not as big of an issue as most people make it out to be... fish will adjust as will the pH... it does more damage to fish if the pH fluctuates a great deal then it does if it stays low or high for a while... Add the carbon filter back into your filter and just do normal water changes on a weekly basis.  clean the gravel really well and if you have time stop by an aquarium store and ask them for "dirty water"  is usually free and is basically dirty sponge filter water... which is full of good bacteria... do a 20% water change and add the dirty water... your tank and pH should correct itself in about a week.  when you do you weekly water change test the pH if its still low... we can look at decorations like driftwood, etc which will sometimes lower pH... hope this helps... dave