A little over a month ago I started a new 29 gallon tank after being out of the hobby for about a decade. I cycled it and then over the course of a few weeks stocked it with 6 corydoras (3 albino, 3 green emerald), 4 plattys, 2 mollies, 8 danios, a nerite snail, and a betta. I was planning on starting the quarantine trio over the weekend anyway but on Thursday last week one of the albinos corys died. I went back to the store and got a replacement on Saturday did a water change and started the quarantine trio. Monday morning I checked on them and 1 molly and 1 emerald green cory was dead. Monday evening when I got home from work an albino cory was dead. And then this morning another albino cory was dead. Any ideas on what could be causing this? Other than the one that died they seemed healthy until I started medicating them. Now even the still living ones seem lethargic (could be do to no food?) and sometimes I'll see them laying on their side when I turn on the light. I did also have that one molly that died but otherwise the other fish all seem to be doing well. I thought it might just be from stress from the move and the medication, but I did have the one die even before I started the meds. I'm sure I'm missing something but I'm out of ideas.
Water:
Temp: 79
PH: 8.0
Ammonia: 0
Nitrite: 0
Nitrate: 5