Why have my two goldfish died?

My girlfriend brought me 2 goldfish, Luke and Lilly, 9 days ago, after 4 days I thought the tank was a bit empty so I went to my local aquarium shop and asked which plant would be the best for them and the shop istant picked one out for me. Before putting the plant in, I did a 50% water change because the water was looking very murky. 2 days later, I thought luke was asleep at the top of the tank in the plant, but after further investigation realised he was dead. I promptly removed him from the tank and cleaned out the tank also doing a 60% water change, in which i added a dechlorinator. I put Lilly back in the tank and she didn't look good as she was lying on the bottom of the tank barely opening her mouth, when usually she opens it wide. I added some disease safe and after 20-30 mins she was looking a lot better. Over the next few days, lilly looked a lot healthier and well. But earlier today, I found her dead on her side at the bottom of the tank, can anybody help me to find out why/how they died?