Dead glowlight tetras

Question
I have had a 20 gallon long tank up and running for 13 months now, and (other than a brown algae bloom once) have not had any problems with the tank.

Tank was populated by:
6 Albino Glowlight Tetras
4 Bronze Cory Cats
1 Otto
2 Dwarf Gouramis (one male and one female)

Two days ago, I noticed that two of the tetras seemed to be having trouble swimming and staying upright. They also seemed to be breathing faster than the other tetras remaining in the tank.  I did not have any medicine on hand (never needed it before) but did add some additional water conditioner as according to the bottle it "Aids in the healing of skin wounds, torn or damaged fins and fish that have been damaged due to injury or disease", so I figured at least it wouldn't hurt.

Yesterday both of the fish had died.  I removed them from the tank and did a 50% water change vacuuming all of the gravel in the process.

No other fish are showing any signs of illness, but it seems very unlikely that two fish would have the same problem and suddenly die without there being a larger problem, so I would like to correct it before I lose any more fish.  I would also like to get another couple of glowlight tetras as I have read that they always feel better if they have a school of six or more to hang out with, but don't want get get them until I am confident they will survive.

Ammonia and Nitrite levels are zero and have been since cycle was completed. Nitrate is usually between 5 to 15 ppm.  20% water changes are done weekly, and filer media is checked monthly (although only half of the media is cleaned or changed at any time). Filtration is a Eheim canister filter. Can't remember the model of flow, but it is suppose to be good for aquariums up to 35 Gallons (an I only have 20G). Ph is consistant at 7.8.

Fish are fed flakes 5 days a week, with sinking algae wafers one day and baby brine shrimp/bloodworm frozen cubes the final day.

Any idea what caused these fish to die, and what I can do to stop it from happening again, or what I can do to help a fish that starts to show these symptoms in the future?

Thanks,

Answer
Hi Allan,
Thank you for all the info you provided! Helps so much!

I'm sorry to hear you lost your two tetras. From what you described to me, my first thoughts were their systems were simply shutting down. I have seen elderly tetras behave this way quite suddenly out of the clear blue. And the problem would not be a disease, but old age. The fact that your glowlights are albino may even cause their systems to be different. Lifespans do vary from a few years to only one or two.

Since this has happened so fast and sudden I'm thinking something may have gone wrong internally. If the fish showed no obvious disease signs, besides stress symptoms like heavy breathing and having trouble swimming, the best thing you can do now is wait it out and see. I'd give the tank about two weeks if you are worried.

So far you have already taken the needed steps when mysteries like this happen. A water change and water testing is always mandatory. Or at least a water change if the other cannot be done.

You really do seem to take very good care of your fish so the best I can recommend is to keep carrying on as you are doing and keep a watchful eye on them.

If another fish starts to show these symptoms I believe they are already too far gone to be helped unfortunately. Lets hope nothing more like this happens in the future.

Best of luck and I hope this helps!
Susan~