Just a quick point about 'toxins' from 'cleaning products', etc. I was actually going to post a separate thread before I read this series. If you consider the volume of 'stuff' in your air, it's extremely small to begin with. Second most of the things in these products are fairly 'non-toxic' in the concentrations used. Unless it's a nano tank, I think the pledge etc is a red-herring. Unless someone purposefully dumped something in your tank - I think you can rule that out - or at least put it way on the back burner.
IMHO - this relates to an oxygen problem or an ammonia problem for whatever reason. We have a couple actual data points - an ammonia of 4 ppm - and a pH drop, and a possible overly stocked, relatively new tank.
I know the comments are going to start 'well it's an API test' - the issue seen with API tests is usually a mis-read of ammonia between 0 and 0.25, as compared to an ammonia of 4 ppm. BTW - 4 ppm could be 6 ppm using that test - depending on the lighting. For example - in your picture - just posted the shading from the light - and the camera makes it (to my eye) an ammonia between 0.25 and 0.5 - and under correct lighting - probably 0.25. Your ammonia could easily have dropped from 4 ppm to 0.25 ppm over the time period - with no waste produced.
Again JMHO. Hope this helped
