Thanks for the additional info.
I’ve got Tetras, Guppies, Corydoras and Cherry Shrimps.
I will explain exactly what is occurring here so you know.
In any aquarium, organics accumulate; fish excrement, any dying/dead plant or animal matter, uneaten fish food...these are organics. As the bacteria break them down, the decomposition process, it produces ammonia and CO2. We can forget the ammonia and deal with the CO2. CO2 produces carbonic acid. The water becomes more acidic, and the pH goes down. This is the basic principle that occurs in every aquarium with fish.
The GH (general hardness, which is primarily the dissolved calcium and magnesium in the water) along with the KH (carbonate hardness or Alkalinity) are connected to the pH. The higher the GH and KH, the more it serves to "buffer" the pH to keep it relevant. In your case, the GH and KH are quite low, so there is very little "buffering" capacity. The pH will thus naturally lower.
As seangee noted, this is not at all a problem provided you have fish that require or are suited to such water. With soft water species, you can ignore the GH, KH and pH and your fish will thrive. Just stay with soft water species. The guppies here are not going to do well, so you should re-home them. You would have to raise the GH considerably to accommodate their needs, and this would then impact detrimentally on the other soft water fish.
It is much simpler to work with your water than trying to adjust the parameters. This can get extremely complex and dangerous. The fish, provided they are soft water species, will be healthy and less likely to have stress and disease. There is absolutely no reason to add limestone or shells or coral or similar; it willnot increase GH and that is the prime factor. Work with nature, not against it, and your aquarium will have far fewer problems.
