The aquarium is not fully cycled, so you will have to do frequent partial water changes until you consistently get readings of zero ammonia, zero nitrite, and positive nitrates. Then you will know the cycle is complete. Make sure you have a liquid test kit, as they are more accurate than strips. Test daily and change as much water as you need to, to keep ammonia and nitrite levels below 0.25 at all times. Any higher than that, and you are stressing your fish and may see signs of distress like you described. Make sure to match water temperature and use a dechlorinator when changing water.
It may take a while, but you will get there. If you want to speed up the process, get some used, wet filter media from a healthy, established aquarium, and put it in your filter.
joy to the fishes in the deep blue sea
55, 30, 30, 20, 10, and 10-gal. freshwater tropical tanks