It's probably too late to help, but I thought I should at least try!
I have a 70L tank that has been setup for roughly 1 year. Freshwater, 3 small/med live plants, driftwood and two broken pot type accessories. Before this incident I had:
5x Black Widow Tetra
6x Neon Tetra
1x 4cm Pleco
1x Clown Loach (young/small)
Not sure of my /current/ water test results as I usually do regular tests at my local produce store. pH is neutral to slightly acidic.
About a month two months ago I introduced 4 small clown loaches (plan was to eventually move them to my 6ft tank with my Pleco when it had finished cycling and proven safe/etc) Anyway, silly me- I didn't quarantine the loaches for long enough and boom~ I had Ich. Managed to get right on top of the Ich before it killed many (took 3 Loaches and 1 Neon) and since then I've been monitoring the tank very carefully to make sure it's 100% safe before I added any new fish or moved anyone out into a different tank.
Anyway, yesterday I arrived home from class- fed my fish and checked everyone out then went to work for a few hours. At roughly 6pm suddenly my Pleco was dead and a Black Widow Tetra was swimming oddly. I scooped out my Pleco and thought maybe it was just one of those things. 3 hours later 2 of my Black Widows had black around their mouths and they were all gasping for air. I did a water change and upped my air curtain. An hour later 4 Neons were dead, 1 BWT was dead and the others were all swimming strangely. My Loach was hiding and refused to come out.
Another hour later and the only fish left in my tank were 3 BWT and my Loach. I checked all the fish out carefully- no signs of anything external other than my BWT having black mouths (which they didn't have a few hours ago) I put my air curtain on max, double checked my filter and heater were working properly and put in some Bactonex.
In the morning 1 more BWT was dead. I now only have my Loach and 2 BWT.
24 hours ago everything in my tank looked normal- everyone was eating, no signs of anything; no fish looked lethargic, ill or gasping. Still no signs I can tell you other than the black mouths on the 2 remainig BWT. My loach looks normal, though he seems quite stressed as he's hiding when normally he hangs out at the middle section of the tank.
I guess I'm more wondering what could have happened rather than what I can do at this point. Not sure I can do much other than wait and see now.
Edit: I do a weekly water change of ~25-30% using rain water aged over the week.