PH is low, nitrates were high. I only add two fish at a time.did you check the PH? is the temp where it needs to be? did you have a mini cycle spike? what were the sexes in there? did you acclimate the fish before putting them in ether by temp (Floating the bag for a while) alone or temp and drip? few questions
(After though) you said that you did a fishless cycle, did you do a huge water change after and big gravel vac to get the Nitrate levels down after? Most of the time after you do a fishless cycle the Nitrates go sky high since you didn't have to worry about the water going to toxic levels. What are the Nitrate levels at now? how many fish did you try to add at once?