Warning: Sad news follows.
Sadly I have been losing harlequins at around 2 per day, all test results come back fine using a liquid based test kit, fine as in 0 ammonia and 0 nitrates with a very very small number of nitrites.
The first 2 happened overnight, the next day 1 was acting very strange, swimming fine but then suddenly looking unconscious I seperated him into a small spare tank I have but he didnt make it, then another 2 during the night, the next day was another wierd one, one of the harlequins mouth seemed to have siezed up and was whitish, i put him in the other tank also but same result, I have 4 left who so far seem ok, no signs of any disease or sickness, I wonder if I just got a bad batch from pets at home? the oto's shrimp and gourami are all fine....
I introduced them the way I have been taught, flot bag in water for 15 minutes, then open the bag and let it float at the top slowly adding tank water for 30 minutes then add using net not the bag water, even used some of the api stress coat +.