good means 0 ammonia, 0 nitrite, 5-20ish nitrates
tank has been running 4 months
79 degrees heated
change 20% every week
If your plants are growing properly, you should have little or no nitrates. What plants do you have? How often do you feed (the plants and the fish) and with what.
I'm too much of a rookie in this forum to know what "cpd" is, but it sounds as if you added three times your initial bioload twice, and with very active fish that do better with more room. IMO, that is not a good idea with any tank, especially one as small as yours. And please don't feel that I'm bashing the size of your tank...MY first one was a 10 gallon. And no plants. That was some decades ago.
Taking care of a small tank is MUCH more difficult than working with a larger one. For example, you say that your tank is "79 degrees heated," but there is no way to know if your tank is staying that temperature 24 hours a day. It could easily drop in temperature overnight or increase in temperature rapidly once the lights come on. While guppies are known to tolerate such conditions, few other fish are, IME.
If you have the space, I would strongly urge you to go up to perhaps a 20 gallon tank. Sadly, such tanks, and many larger ones, are very commonly available in the want adds (shows my age) or Craigslist or some such. You could do so with little outlay, I think.
Finally, don't lose hope because you've lost some fish. While no one enjoys fish dying, if you haven't lost any fish in your career, you haven't kept them more than 5 minutes. I've even lost some on the way home from the LFS across town!