I'll try to answer everything from the last few posts.
GH is general hardness, and KH is carbonate hardness (often called Alkalinity). We really must have these numbers for your tap water, as they are probably the cause but without them we are guessing. GH and KH is not something most aquarists fiddle with, as it is safer to use what comes out of the tap and select fish that will manage. You can by a kit for GH/KH, but before spending money let's make sure you will use it. Once we know the GH and KH numbers for the tap water, we will know what we have to work with.
On the pH. When testing tap water, you must out-gas the CO2 in order to get a more accurate reading. You can let the water sit 24 hours, or you can shake it briskly for several moments; either way the CO2 will dissipate. CO2 adds carbonic acid to water, thus lowering pH. Tap water usually has some amount of CO2 dissolved in it, this varies from place to place. Also, only use one test for both because you can get different numbers depending upon the test, as you've seen. Once we have this all sorted out, the appropriate test for the tank will be obvious.
To explain why the pH is lowering in the aquarium. This is natural, due to the buildup of CO2 from fish and plant respiration and the breakdown of organics by bacteria. The CO2 creates carbonic acid, which lowers pH. Now. the extent to which this occurs depends upon several factors. First, the tank biology; more fish in smaller volume of water means more waste and thus more acid. The more food that goes in, the more waste. The larger the fish, the more waste and CO2. But aside from this, there is the KH of the source water. KH acts as a buffer to maintain the pH where it is coming out of the tap. The higher the KH, the more it will buffer.
My situation to illustrate. Tap water is near-zero GH and KH, and pH is 7 to 7.2; in my tanks, after a couple weeks new, the pH will be 5 if i do nothing. This is fine for the fish I have. In two tanks I buffer pH with aragonite to keep it in the mid 6 range; the other tanks I let go. The fish in these respective tanks are fine with the pH.
So, I am expecting your KH to be low, and usually the GH is similar. The water folks are probably adding something to raise the pH, assuming the GH is low. Mine do; they do this to maintain a higher pH which will not corrode pipes and water tanks as much as will an acidic pH. This has no effect on GH (or on KH either, we may assume from your pH numbers) and it is harmless. But worth knowing, as it is part of the whole picture.
Last comment on the fish adapting to your water. This is a commonly-held belief by some, but one I do not accept. If you read my article on Stress http://www.tropicalfishkeeping.com/f...um-fish-98852/
it goes into detail on the ways the physiology of a fish is designed for specific water parameters, and any variation will cause issues. Not everyone wants to hear this, but it is a fact of science. Now, having said that, there are some fish that can adapt to some extent to different parameters, but this has its limits and is also limited to specific species. When it comes to livebearers, they must have mineral in the water or they will not be healthy. Soft water fish are the opposite; hard water for them causes calcium deposits in the kidneys and other issues. Fish maintained too far out of their preference--which nature has programmed into them through evolution over millions of years--will not be healthy, their immune system is compromised, and in all cases they have shorter lifespans as a result. The cited article will expand on this.