Well, my guy didn't make it.
It is strange to me that they just die off one by one over time. It seems like if there was something wrong with the tank they would all get sick at the same time.
The first thing I did was remove the pvc. Which is sad because the tetras really liked it.
I hadn't done anything that would cause a spike. My filter doesn't have media that needs changing. It is an in hood filter that has a combination of gravel and things for the water to travel through. I could add some activated charcoal to it for some chemical filtration.
The larger rocks came with the aquarium. I bought it used. They are all smooth and the same size so I assumed that they were made for an aquarium.
I'm so sorry about your fish :(
My opinion is the rocks are still questionable (although they look sort of like slate which is inert for the most part). I say this because I bought some supposedly safe aquarium rocks at petco a couple months ago. Well once they were in the tank, one of them started turning orange in the cracks of the rock - it was rust (aka iron). I took it out. So even some aquarium rocks aren't all that safe. just fyi!
I understand your point about all the fish not getting sick at once. If you have water issues, esp if they are minor issues, the fish won't all necessarily get sick and die at the same time. It just depends - some are stronger and can take more poisons than others.
Im not familiar with that type of filter so I can't comment on that (what brand is it?). Early in my aquarium experience, I had a similar problem. I realized that my filter was not equipped to handle the waste that was generated. This in turn messed with the water and weakened the fish, and one by one they died. (By messed up water I mean bacteria and parasites specifically, not just ammo and nitrites.) I almost gave up, but I didn't, so don't give up yourself!! Generally speaking, I think most folks on the forum advocate for a filter size double the aquarium size in gallons. So for instance I have a 50 gal rated filter on my 20 gal tank, someone else might have a 70 rated filter on their 35 gal tank, etc.
If I were in your shoes, I might try dosing the tank with an antiparasitic medication (in case maybe gill flukes) and/or a broad spectrum antibiotic, HOWEVER YOU WILL LOSE YOUR BIOFILTER so I'm not advocating that you do this. It's just what I would do if it was happening to me. Or if you have a quarantine tank you could take the fish out and dose them then return them to the main tank afterwards. I can understand your frustration with this, I sure hope it gets better from here on out.
Maybe one of the other people on the board will have other ideas, but for now, that's all I got!