Came home yesterday to find 11 of the 27 fish in my rio 400 dead. 10 of these were the entire school of denisoni barbs. 1 was a kutubu rainbow. The other fish that survived were rainbows, a RTBS and a SAE. I am suspecting that it was oxygen depletion although not 100% sure. I have taken co2 off for about a couple of weeks although there is still considerable plant mass (im in the process of setting up a larger tank). So i think CO2 build up is still possible in the night, plus the higher temps over the last week or so (reading 27c). I had conducted a 40-50% change of water and had also added some melafix and pimafix as one of the rainbows seem to have a small infection on the mouth(nothing unusual or major, common in many rainbows). However im thinking that maybe these could lower the O2 level? I had this situation about 2 years ago when i lost my entire stock. Think now I will add an aeration pump to the new aquarium that will come on at night time. checked water parameters, seemed ok although nitrates around 30ppm, this could have been from the water tap water which sometimes has this level of nitrates. Have not dosed since removing the co2. I am guessing that maybe the medication may have tipped the balance, im not sure though, plus doing a water change one might think that that would help aleviate any oxgen depletion. However the fish lost wad almost all just denisoni barbs and not the others. Very sad, any other ideas?