Disaster in a 55 gallon tank

I worked from home this past Wednesday, following my normal routine – get up early, go to the gym, come home and get on a conference by 8am.  Somewhere between calls, I make the rounds to turn on my aquarium lights and feed the tanks.  When I walked into my family room and turned on the light, I saw a horrific sight…  Nearly all of my fish were laying dead on the bottom of the tank.  That was heartbreaking.  My prized 4 year old angelfish, named “Tailfin” was among the dead.

There was no indication of what happened – the fish were fine when I had turned the lights out the night before.  3 young angels and a pleco survived, so whatever happened could not have been totally devastating.

I perform 50% or larger water changes weekly, and have for years.  I vacuum the gravel thoroughly each time.  It had been 3 days since the last water change.

Here is my theory on what happened:

The day before, we lost power in our house for about 20 minutes.  Not terribly long.  I have battery powered air pumps that turn on when the power goes out.  All of them came on, except one – the one in the 55G tank where the fish died.  I replaced the batteries, but it didn’t work.  I noticed that if I took the hose off, it would very slowly work.  I hooked the hose up again and had to go take a phone call for work.  In the mean time, the power came back on.  When I finished the call an hour later, I walked into my family room and the battery powered air pump was pumping normally – because I had forced it on whilest trying to get it to work earlier.  I turned the pump off and didn’t think about it until the next morning, when I found all of the dead fish.

Upon inspecting the air pump, there is a think tar/oil like liquid that appears to be oozing out of the air outlet.  My suspicion is that water from the tank had somehow siphoned through the air line back into the pump where it sat for many months.  The reason the pump didn’t work at first was that it was gummed up with this likely concoction of water, oil, rust, etc.  Eventually, the pump pushed the goo out and up the air hose and into the tank.

What I can’t reconcile is why the fish did not start having trouble the night before – 9 hours after the pump was turned off, and presumably after the goo was put into the tank.

So, that was extraordinarily sad for me, and I will try to learn a lesson from it.  I have decided to re-populate the tank with African cichlids, using the opportunity to try something new and challenging.

I will post pictures as I can.