Like everyone else, it seems a bit of a mystery to me.
Maybe clutching at straws a bit but once you get to a certain amount of fish in your tank, that is generally it and if you add any more, something usually affects the fish. In other words, a community aquarium usually finds its own stocking levels.
I just wonder if your tank was quite happy chugging along with the amount of fish you had and when you added the swords, it's tipped the balance over the top, the filter couldn't cope with the extra bio-load and you've had an ammonia or nitrite surge.
That said, you would usually expect the clown loach to suffer first as they tend to be very sensitive to water changes.
Personally, i'd give the tank a 50% water change straight away and don't feed the fish for 2 days and then go from there.
Please let us know what the outcome is.
Good luck