I had aquariums in the early 1970s. Never had the problem with dying fish that I am having now. When I set up my aquarium about 2 years ago I immediately had a algae problem, but my fish seemed to thrive. I changed the water just like i did in the 70s.

Then I put too much Excel in and killed all of them except the Cory's. I finally figured out where the extra light was coming from. Now I have sparkling water. Back in the dark ages I changed about 1/4 of the water every week. Books I have read say to change 1/4 every month. I changed to monthly water changed about 2 months ago. I had one guppy and bought three more. One died within just a few days. The one I already had died several weeks later. And now I found another dead one this morning. The last one looks like it will be gone any time. I changed water today and decided to go back to weekly changes.

I have a 10 gallon tank and other than the guppies, I have a Cory and an algae eater so I don't think it is overstocked. I know I should already have tested the water, but every time I have checked it its been well within the numbers. Other than not changing the water often enough or water unbalanced, what else could be killing the guppies and not the other fish?