Ammonia False Positive? SOLVED

As followers of Project: Fishy 185 will know, I have a 70L tank currently.

I did a 50% water change on Wednesday and cleaned the filter in tank water, giving the sponges a good squeeze but that's all. I also installed a bag of Seachem Purigen.

I tested the water just now and got ammonia readings of at least 2ppm! So either my fish are in clear and imminent danger or something is giving a false reading. Unfortunately I didn't test after the water change so have no point of reference, but even if I destroyed all the bacteria (which I surely didn't) 2ppm in less than 48 hours in a tank with a 50% water change?? Surely not.

My fish appear fine and healthy - and are eating well. They are all active and in no signs of distress. Even injecting 1-2ml ammonia solution into my new tank didn't produce readings this high!

I used a clean syringe. I used a test tube which had been sat in tap water but was relatively dry. Could this give a false reading??

Any trace of chemical in the tube has the potential to distort the reading of the next test.
I've seen a video of one manufacturer saying that you should never use a tube to do more than one kind of test; the video showed how to mark each tube so it was only ever used for one type of test. To my mind, that is a bit extreme - provided the tubes are washed properly between tests.

I use the technique we used in research labs at uni. Empty the tube then rinse a couple of times in cold water. Then half fill and shake, repeating three or four times. Here I diverge a bit as I don't have supplies of acetone to rinse the water out of the tube, so I use a paper tissue to poke down the tube to dry it. If the tissue shows any staining at all (easily seen on white) I wash the tube again. If necessary, use a small bottle brush.
I also know from experience that if tubes are washed immediately there is less residue stuck to the glass.

I have learned recently that we are supposed to take the test sample from at least three inches below the surface as debris floating near the surface can interfere with the reading. This applies to all tests.

And of course there could be a blip in the test procedure. Did we mis-count the drops? Was one of the drops a bit smaller/larger than the others? Etc etc.


And if I get a suspect reading, I test again using another tube.

Cheers, guys.

I used paper towels to clean and dry the tubes. I could do with a bottle brush.

Interesting about taking a sample from well below the surface. I'll do that next time.

I just can't wait to get the fish from the 70l into the new tank! It will be so much better for them.