It is important you check your city's tap water for ammonia and nitrites. In addition look at the city water report and see what they say about the average and high and low PH.
Our city (Topeka, KS) tests positive for Ammonia at 1.0 to 1.5 ppm. This was not the case back in the spring when I found no excess ammonia. After multiple emails and 'ccing their Legal department I got a call back from the City water manager who admitted that there should be NO leftover ammonia in the water after using it to treat for chlorine. He said he would contact his Water manager and find out what is going on - in the meantime he suggested I drink "bottled water" even after I explained this was a FISH TANK issue (my Brita by the way, doesn't get rid of the ammonia so it's still in the water I'm drinking, not that it particularly worries me - I'm more worried for my fish tanks).
So this means every time I do a water change to reduce or eliminate ammonia, I'm adding 1.5 ppm ammonia back in, thankfully Prime seems to take care of it. I recently cycled a 29 gallon tank and was able to get (and keep) my Ammonia under control. I now read zero Ammonia and zero Nitrites. Will be doing another water change in a few days (I ran out of Ammonia test solution and the one I ordered from Amazon is taking it's sweet time getting here).
As an aside, the average PH for Topeka water is 8.9 with a range up to 10 or 11. Before I add fish I HAVE to lower the PH to get it closer to 6.8-7.0 for most of my fish. While many object to using PH- or PH+ - I have no other choice - the so called "natural methods" you have NO control over and it pushed my PH down to below 6.0, so I had to purchase PH+ to get it closer to 7.0. Adding 2.5 mls per day it takes 2-3 days to get the PH up and I have had minimal rebound due to Hydrogen ions in the water. Once I get the water adjusted it usually stays that way, even with water changes (unless I have to do daily water changes and then my PH will go up into the 8's due to my city's tap water). Unfortunately I have to add this with the fish in it but I have never had a single fish die from altering the Ph over a period of 2-3 days. I know there are many people that object to doing this, but again I have no choice and it's been safe and successful for me so far.
Still waiting for the City Water manager to get back to me - I have his private number so I'll be contacting him soon.
I would also advocate doing larger water changes than 25% - I don't believe that little of a water change is going to help with any ammonia or nitrite issues. Aim for 50-75% and you may find (once you start to test it) that you have no ammonia or nitrites and may be able to get away with water changes every 2 weeks if you don't over-feed.