Nitrate at 80 ppm is high, but probably not the cause of death of your fish. Because all died at one time, and your corals and inverts seem to be okay, I can only think that the fish were all sick and you couldn't tell that they were. Or, perhaps something toxic got into the tank that only affected the fish. Gads, there's nothing worse than losing fish when you have no idea what caused it
I would like to offer a few maintenance suggestions:
1. Do weekly water changes of at least 15%. Yeah, it's a PITA at first, but once you get into the groove, your tank's inhabitants will really appreciate your efforts
You will also experience fewer nuisance algae problems in the long run.
2. If you have any mechanical filtration, thoroughly clean the media in changewater every week. Doing so will preserve the bacteria, but remove the crud buildup that helps cause high nitrate and phosphate levels which cause nuisance algae.
3. Check alkalinity, calcium and magnesium levels at least every two weeks. Maintaining these three important chemistry components in your 20g will help your corals thrive instead of struggle along. Here are some links to help you understand reef chemistry and how to correct imbalances:
Aquarium System Volume Calculator:
http://home.comcast.net/~jdieck1/volcalc.html
The Reef Chemistry Calculator:
http://home.comcast.net/~jdieck1/chem_calc3.html
Reef Aquarium Water Parameters:
http://reefkeeping.com/issues/2004-05/rhf/index.htm
Magnesium in Reef Aquaria:
http://www.advancedaquarist.com/issues/oct2003/chem.htm
Solving Calcium and Alkalinity Problems:
http://www.advancedaquarist.com/issues/nov2002/chem.htm
Sorry to hear about your fish
Hope the above suggestions are useful to you
