My boyfriend was given thousands worth of Goldfish and Neon Tetras

My boyfriend was given all the things for a 30 gallon tank from a neighbor that couldn't keep them. They had been doing well for the first while, but a week or so ago the Goldfish and Neon Tetras started to drop like flies. We returned all of them to the pet store and were never told they weren't going to survive.<br><br>When he got the tank, he was told that he needed to change the water and clean it often, but he didn't know the frequency. We have Goldfish, Neon Tetras, Zebra Danios and White Cloud Mountain Minnows. We're wondering why the Goldfish and Neon Tetras were dying off while the other two have been doing well. We did a water test and found that the pH was high. We made the mistake last week of doing a 50% water change, thinking that we could rapidly change the pH. We then did a water test before doing a second water change and found that the pH was within the neutral range. The ammonia was high, but we have a filter running, but we have heard that it takes a while for the ammonia to be neutralized.<br><br>Last week, on Friday we did a 50% water change. On Tuesday we did a 10% water change. Today, we did another 10% water change. We are wondering what would be the best frequency for water changes for our tank. Should we be doing 10% on a daily basis until the ammonia is gone or should we do less frequently? Any tips for someone who is new to breeding would be helpful.<br><br><br><br>TL;DR: Received a 30 gallon tank and the Goldfish and Neon Tetras have been dying while the other three species have been thriving. We are trying to figure out the best step to help them survive.