Fresh water tank crash

[asylumdown]

Hey folks,

Been a while since I've checked in here. Hopefully someone can help me trouble shoot the cause of a recent melt down of my 40 gallon.

It's at my work so I only see it during working hours and if I come in on weekends to take care of it, otherwise I do most of the maintenance over my lunch break. Over the past few months I will admit that my maintenance routine has fallen a little behind as I now have two saltwater tanks at home that get most of my my time in my off work hours, so instead of getting water changes every 7 days, it's been more like every 10-12 days the past few months but until recently everything was doing fine.

The tank:
Fully planted and mature, it was set up about 14 months ago. Large gravel base with finer gravel spread on top for the plants to root in. Filtered by a Marineland C-220 external canister. It's doesn't move much water so there is a small koralia in the tank for extra circulation.

Temp: 81.5 F
Fish: 2 Boesemani Rainbows, 1 Blue Diamond Discus, 1 Yellow pigeon blood discus, 2 flying foxes, 2 praecox rainbows, 8 cardinal tetras, 3 oto cats and 5 ghost shrimp.

I know it's an odd mixture of fish, but that stock list hasn't changed in about 7 months so everything seems to have adapted to each other and the slightly higher temp for the discus.

The Crash:
Tuesday: at lunch I did a thorough clean, thinned out some over grown plants (and used the opportunity to do a massive vacuum of the gravel that I couldn't do with the plants in the way), did 40% water change and cleaned the filter (using tank water to rinse all components, like always).

Wednesday: everything was fine, fish were happy, water was clear.

Thursday: everything was fine again, fish showed no signs of stress, left work at 5:45 and all equipment was working normally.

Friday morning: both Boesemani rainbows, both discus, both flying foxes, 1 praecox rainbow, 5 cardinal tetras, 1 oto and all 5 shrimp were dead. The surviving praecox rainbow and the cardinal tetra's were clustered near the surface gasping for air. Unfortunately on Friday I was working off site so I got the news via phone call and I had to immediately leave town for a function over the weekend so I couldn't come in myself to see what had happened. The person who discovered them 'thinks' the filter was off, so he unplugged it and plugged it in to a different socket and the filter came back on, but the filter was plugged in to a working socket of a power bar so I can't imagine why it would have died. The temp was a solid 81.5 degrees friday morning and the heater doesn't appear to be broken. I wasn't able to come in before I left town and the person who found the tank forgot to take a water sample for me to test (grrrr). I tested the water today and other than a slight colour change on the ammonia test (the guys at the office removed all dead fish except one flying fox, which rotted in the tank all weekend) and unusually high nitrates, nothing appears to be out of the ordinary, though with the filter running properly all weekend an ammonia spike would have been cleaned up by now I suppose.

Does anyone have any ideas as to what could have caused that dramatic of a crash? Even if the filter did somehow mysteriously fail after the office closed (it appears to be working just fine now), I'm having a hard time getting my head around the idea that 12 hours (max) with no filter would kill EVERYTHING, there has to be at least some filtration happening within the substrate, on the plant leaves and on the driftwood in the tank and the koralia never stopped circulating water. Could my aggressive gravel vacuuming a few days before have started some sort of reaction? All of these fish had names and the people in the office were pretty devastated to have them all die like that so I'm nervous to restock the tank without a cause.