Urgent Guidance needed....?

I have a 55 gal tank and it homes two Koi fishes and a shubunkin. The Koi are 3 inches and 2 inches each. The shubunkin is 3 inches. I know that Koi is a pond fish but I happened to bring them home without knowing about them. I am planning to rehome them once they become 5 inches. The tank temperature is constantly maintained at 72F. There is a filter running 24/7 with a 1 feet aerator / pump ON 24/7. The tank is also filled correctly with gravel at the bottom. The fish were all happy but I noticed in the last 36 hours, they have become dull. I did a 25% water change but still they are not as active they used to be. I did the regular test kit tests. This is what I found from the test. pH is between 8 and 8.2, Nitrite is 1.0ppm (mg/L), Ammonia is between 4 and 8.0 ppm (mg/L) and Nitrate is between 5 - 10 ppm (mg/L). Now I thought fish keeping is pretty simple and easy and should not take much of my time. I brought them because my 4 year old son happened to just ask me for a pet and we happened to get them from Walmart. Now it looks like more than my take being taken away, I am feeling bad for the fish as may be I am unable to provide them better environment consistently. Where ever I read, it is told Koi and Shudunkin are easy maintanance fishes and nothing to worry about. Now my questions. 1) What are correct nos that should be maintained for both Koi and Shudunkin in terms of pH, Nitrite, Ammonia, Nitrate.? 2) Am I doing something wrong in maintaining them? I do remove the extra food in the tank after 30 mins of eating time. The tank is not flooded with things. 3) I am sure the reading that I have mentioned are off the limits. Now what should I do to fix this? 4) I have the master test kit for testing the water. I am ready to test them weekly once and even change the water 25% every week. I am ready to remove extra food daily with a net. Amidst all these things, still if the tank is going to be off the basic vital signs, then I am not sure whether I am up to keeping fish as a pet. Please guide. This is really urgent and importnant for me. I dont want to hurt the fishes and also hurt our family too by allowing them to die.