We received a 10 gallon aquarium as a gift. I set it up and took my daughter to buy fish. I did ask the pet store employee many questions such as "Can this fish go with this one" and such but I was told how easy it was and not to worry about it as long as I don't get certain fish (goldfish, betas, some others). My brother is an enthusiast (and might even be a member of this site, I don't know). I can't remember what brought it up, but about 2-3 days after, I mentioned the new fish to him.. he said, "You just put them in after setting it up? Do you know anything about the nitrogen cycle?" Me - "Um... wha?" Needless to say, I was an idiot, did no research, did this all wrong and now I have live fish that I need to save. After research and testing the water with my new API Master Kit, I figured it would be best to move them a new tank because the ammonia was just sky high (we were also overfeeding them). They are now in a pretty 20 gallon tank with a big log decoration, several fake plants, a mix of gravel and sand substrate, a Tetra Whisper 20l filter, and a 100w heater. Everything was rinsed (the gravel was boiled and rinsed) before putting in. Then I added the water with water conditioner and a bottle of Tetra SafeStart. I waited awhile and then added the fish. Here are the fish I have - 1x Diamond Tetra, 2x Swordtails (male and female), 1x glo danio, 1x ghost shrimp. I know, the tetras need schools, and etc.. the store employee did not mention this to me and I did not research. I will fix that problem later, if I can keep them alive. I am now going to test the water every day and do water changes if the ammonia is too high. I'm also going to feed very sparingly through this. What else do I need to be doing and what changes do I need to make to my tank? I hate the thought of killing animals because I was thoughtless. Thanks in advance!