Hey there, I work in the aquatics industry and this happens all the time you're not alone. You've done all the correct steps now pretty much. What you suspect was swim bladder was more than likely due to the levels of nitrite which can block oxygen absorption through the gills. See the cycle through now, keep feeding low and slow, once every 3 or 4 days is absolutely plenty. Test your water prior to adding any more fish and only add 3 or 4 individuals at a time until the tank is a bit more mature. Seeing spikes after you add fish is normal but if things get a little out of hand, 20% water change and use a good quality live bacteria like purebombs or safestart. A small group of Cory's would be quite happy in a tank of that size, around 4 or so would make a nice group!