Were it me,, (and it ain't) I would read up on starting a new aquarium so that i was familiar with the process. I would then purchase a test kit .API freshwater master kit,is the one most prefer.The test strips are notoriously inaccurate. I would then purchase a good dechlorinator such as PRIME that detoxifys ,ammonia,chlorine,and chloramines.
I would then set up the aquarium,put in substrate,fill with dechlorinated water,place filter and heater in the tank and let the tank run with no fish for one week.Would also be a good time to place plants in the tank.
After a week ,assuming everything is running and no problems with equip ,and temp is right, I would purchase no more than three of the zebra danios. I would leave these three small fish in the tank for ten days ,feeding them a tiny amount of food every two days no more. These are small fish and if not overfed,, will not create excessive amounts of ammonia which is good. They also represent a very small bio- load. Your tank will begin to mature naturally. After ten days,, I would purchase no more than two more of these small fish. Again, you will be feeding them sparingly so ammonia levels will not become dangerous. Your tank will continue to mature or (cycle) naturally. You will need the test kit mentioned to occasionally (every couple days) test the water to ensure that the ammonia levels are not above dangerous levels which will be indicated by results of water test. (test kit is easy to use).I would leave these five fish in the aquarium for another ten days while feeding small amounts and testing the water every couple days. If ammonia levels rise to dangerous levels according to test results,, You are feeding too much and water change of 10 to 20 percent will be needed any time levels become dangerous. I would place a small amount of food (about dime size) in the palm of my hand and then crush it into almost a powder. I would sprinkle a tiny amount every couple days in the tank and watch to see that the fish eat it. If so, I would add a tiny bit more. Again,, feeding this tiny amount every couple days to three days, will help keep ammonia levels from becoming dangerous and fish won't starve. By now you will have had fish in tank for approx twenty days .I would not add any more fish for another ten days. I would not clean or replace the filter during this month ,nor would i disturb the gravel. You will not have been overfeeding and the tank has been maturing naturally and slowly and you can still enjoy viewing the fish. At the end of thirty days ,I would perform a 20 percent water change using dechlorinator for the new water. I would perform this 20 percent water change each week from now on being careful not to give fish too much food ,,but perhaps ,begin feeding them once a day. I would clean the filter material in old aquarium water during this first water change and stick it back in the filter .When the material began to fall apart,, i would replace it but not until then.
This is a method that was shared with me and I have used it many times ,perhaps,others have as well. It is the way I prefer but may not be what others prefer. If fish are small and few,and not overfed, I have found that frequent water changes are not needed and test results indicate no dangerous levels of toxins. IF too many fish are added,too large of fish are added,or fish are overfed, This method will not be successful and a very real probability exists that fish will be subjected to lethal levels of ammonia,and nitrite poisoning as well. But if done slowly as i have attempted to explain,, The tank will mature or (cycle) without incident.
Others have other methods but as stated,, This is the one I prefer. If done properly,, no frequent water changes are needed,and more importantly fish are not subjected to dangerous or lethal levels of toxins.Good Luck!