Some water conditioners deal with chlorine and claim to adress chloramines . Chloramines are or is a combination of chlorine and ammonia. those water conditioners that claim to detoxify chloramines do so by rendering the chlorine harmless but they leave the ammonia for the biological filter to deal with. This works well with healthy bacteria found in the filter but in new tanks where the bacteria has yet to develop ,or where biological bacteria has been destroyed or removed,then nothing is breaking down the ammonia and fish will be under considerable stress depending on the stocking levels or ammonia being produced by fish or over feeding.
Is always a good idea to use a full function water conditioner such as the AMQUEL+ or PRIME for this reason. You may leave tapwater out for 24 hours and chlorine if present will dissipate, Chloramines which are used in many water supplies,will not dissipate. PRIME water conditioner will also remove heavy metals that may be present in the water supply.
It is possible that you have a low carbonate hardness or Kh that although you have a pH of 7.4 to 7.6 from the tap,.the hardness is lacking and thus the ph will fall. Adding pH adjusting powders or chemicals will have little effect for very long and the pH will fluctuate wildly. Will be better if you can determine what your ph from the tap is and also the kh. In this way we can best determine the best way for you to produce a stable enviornment.
Would also submit that if the water from your tap is going through a domestic water softener that this too can affect the ph and kh by removing those minerals and salts that act as buffers to prevent ph from fluctuating. If this were the case, I would want to use water from a spigot on the outside of the house or apartment that water does not pass through a domestic water softener.
For now,I would use the tapwater with full function conditioner and small 20 percent changes every couple days to try and remove some of the regulator (pH) and or anything else that may be affecting the readings you are getting. Then I would try and get testresults for kh and we can go from there.