Sorry this is a bit of an essay...
I have recently had white spot in my tank, and used 1 standard dose of Triple Sulfa. This wasn't effective so a few days later I used the 2x dose recommended for severe infestations. This helped more, but not 100%, so I am increasing the temperature to 30*C for 2 weeks (should have done this first instead of used the Sulfa, but too late now).
Anyway, I change 1/3 of the water every 7-10 days and test the water before every water change. The tank has been set up for 10 years, its had a few cases of disease during this time but all has been great for the last year or so.
Two days after using the 2x Sulfa dose I tested the water and panicked when I suddenly had 1 ppm ammonia (API salicylate test kit). I have changed 1/3 of the water 3 times over the last 5 days. I tested again and its still 1 ppm.
I thought the antibiotic may have killed off my good bacteria (it probably has), but I would have throught I would get a decrease in ammonia after these changes. I tried using a Nessler test and I got a reading of 6 ppm.
Thing is, I don't believe the tests. I have rummy nose tetras and I'm sure they would be very stressed if the ammonia was this high for this long. All fish look healthly besides the white spot which is steeply declining.
I suspected the triple sulfa may be interfering with the ammonia tests (feasible, I can see how monochloramine could be formed from the sulfonamides which could then form the green 5-aminosalicylate). So I got tap water and tested it (no conditioner added) - 0 ppm ammonia. Then I got tap water and added some Triple Sulfa - 2 ppm ammonia reading. Conclusion - triple sulfa intereferes with ammonia tests.
What I am confused about is why, after all the water changes, am I not decreasing the sulfonamide concentration? Maybe ammonia is increasing as sulfonamide conc is decreasing? I am worried because I don't know what the true ammonia concentration is, and whether I should keep changing 1/3 of the water every day.
Has anyone else ever encountered this issue with medication and ammonia testing?