Assuming everything is being measured correctly (e.g., no bad test kits), and adding the post from June 26 (?) you made...
From the post you made at the end of June (where you said you had been measuring ammonia and nitrites and they were always zero), we can guess that 1) your tank was cycled or 2) your measurements were in error. A test for nitrates (at that time) would have been the test that would validated a conclusion. I don't see that you posted having tested nitrate then or since.
However, it would still be great to see what your nitrates are today. IMO a tank isn't certifiable as cycled unless a significant and growing amount of nitrates is detected (barring some other explanation for where nitrates may be going, e.g., plants.)
Your OP now is that nitrates jumped when you added fish. How many and what size fish did you add? How much and what are you feeding them?
That would be perfectly reasonable IF you had increased the bio load enormously "when you added more fish." Nitrosomonas bacteria multiply much faster than nitrobacter and both multiply differently under varying temperature, salinity, ph, etc.
Despite that, the only explanation I can conjure for why the nitrite "has never gone down" is because food continues to be put into the tank. If nitrites are at .25 all the time, but the fish are being fed, then the amount of food is too much and needs to be reduced until the BB catches up. There must be nitrobacter in the tank, otherwise nitrites would be far higher than .25.
You indicated that you "keep" your pH at 7. I take that to mean that it isn't always at 7, since it's normally not possible to make the pH be exactly one number at all times. If the pH routinely goes below 6.0, for example, the BB cease to reproduce and to process ammonia or nitrites, while at 6.5 they are much less efficient.
The optimal metrics (from 2 studies I found) to grow BB is 7.8 - 8.5 pH and 59-77 F. When conditions differ, it becomes less than optimal. Higher temperatures and lower pH plus a large increase in bioload would, for all intents and purposes, be a mini cycle.
I'd reduce the food level and let the BB catch up with whatever bioload is in the tank.
As for the GH/KH issues, I'm pretty short on info there. I don't recall seeing anyone claim in test that those have an effect on BB. (That of course proves nothing other that I don't if it does or doesn't.) I wonder, not only because BB exists in all sorts of water, but because the water that you have been using before and were using at the end of June (for a month), seemed to be having no effect despite it's GH/KH levels. Still, I will admit I know almost nothing else on the matter. Trying to fix the hardness might help greatly.
