New tank!

[Black8ird]

looking for some help!
I got a new 55 gal tank about 2.5 weeks ago. I set the tank up and put in some decor and let run for 2 days. I then added QuickStart and let run for 12more days before adding fish. I added 3 zebra Dino's and a pictus catfish. Today I found the pictus dead in the corner and I lost a zebra Dino. Scared that I would lose the rest I hit the internet and thought I should do a 75% water change.
I'm looking for any help at all please.
I also had my water tested by petco after the first week and I've been testing with stripes every other day my latest test was last night and I was in the "safe" area on the chart. Please help

 

[FreshyFresh]

Welcome. Get yourself an API master test kit so you can test for ammonia, nitrite and nitrate in PPM yourself and get a quality dechlorinator like Seachem Prime that can detoxify ammonia and nitrite temporarily. I doubt 3 zebra danios and the catfish brought ammonia to a lethal level in a 55g that quickly. More than likely the fish were sick and weak when you got them. It's possible to cycle your tank as is, just keep an eye on your water params (with your test kit) and do a 50% water change every day or three. Once your sure the tank produces nothing but nitrates, do water changes so nitrates don't exceed 20ppm.