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Until 1-2ppm ammonia

BANG goes that theory then, d'oh! Are 2 of them close if so I would discard the one which is different. Other options would be to get some water tested by an aquarium shop and see which are nearest and discard the other(s). I have also come across this with other tests such as droplet tests from different companies so would do as CurleyJones says, and I now do, pick one and stick with that one (at least you should get a consistent), only problem with this is working out which is most accurate to stick to using. With 3 there must be 2 which are closer than one so I would assume the third is best thrown out, or as I say, take them all to an aquarium shop and get the shop to test one of their tanks and then you use yours to see which is the nearest? I always swoosh mine in the tank about 10 times in case there is some unwanted contaminant on the prongs, rather than just dip it in the tank. Simon