Hi sorry to hear about your dats,maybe taking the gravel out took out too much filter bacteria as they cover everything that's in the tank including the glass, so if you removed all the gravel in one go then you would have had a massive ammonia spike especially as there was quite a few fish in there, with less fish it probably wouldn't have been so severe, but i still can't understand why only the dats were affected it's strange, i would have thought the aro would be the first to go ! hope this gives you a few thoughts about what might have happened, it could also happen if you put all them fish in in one go as your tank has only been set up a month and the bacteria couldn't establish fast enough to keep up with the waste being produced good luck for the future Paul