Oh I hear ya. I talk on another forum and it seems like everyday here comes a newbie with fish passing away and a toxic tank. I was one of them.
My 10gl I did the same thing. I lost two goldfish that I intended on keeping. Since reading up on it and learning I was able to do a fishless on my 55gl and that felt good and makes me proud. I learned.
You learned and are learning. That is where a lot of people that are fish lovers get upset. Many LFS will tell you to just fill it up and add hardy fish. All they want is the sale. Some will even let you overstock or get the wrong fish, they dont care they want the sale.
Fishless cycle is popular but at the same time there are many that believe in using feeder fish or other hardy fish to still cycle. It is a choice in the end.
The other issue you run into is lack of information at the point of sale. Many places put down the WRONG size of a fish. Like a Clown Loach being 6" adult size. LOL They get HUGE!!!! Columbian Sharks being 6" at adult size. Many get over 12" and as they mature they need to switch to brackfish and as adults they have to switch from brackish to full out marine or else their skin has problems.
Goldfish are kept in bowls when they can get huge and outgrow a 10gl tank no problem.
I am here because I love catfish and you will see that many catfish are misunderstood as well. Plecos are very abused out there because of the lack of information. Many of them get huge. Many people want a sucker fish for their smaller tank and get a common pleco and get shocked it grows so large.
The lack of information has lead to many fish starving to death. Throw a pleco into a tank and assume they will just eat the algae and it is enough and watch your fish die from starvation. Many snails starve to death as well from this belief all they need to do is eat algae.
They do not tell you that these little guys will eat veggie or other foods you can give them.
The only way to know for SURE is to read and read more and educate yourself. Read fish profiles and you will see even those vary online. After reading several pick what you have seen over and over. The profiles on this site I have found to be honest and very educational about the fish.
Join forums and talk to other owners of the same fish you have or fish you are interested in.
My LFS is great and I have two guys that are amazing that work there that have helped me A LOT and they supported my fishless cycle and answer my questions and I ask them again online and get a lot of the same advice.
One day I stopped in and bought another Spotted Pictus and came home with it. (the usual fish guys were not there and some kid got it for me) Thought the color looked funny on the fish, it was a odd pattern and the color was more gold compared to silver, the whiskers were dark instead of white. I thought it was a fluke. I later bought a four-line pictus and realized my other one looked the same. They had previously sold me a four-line when I asked for a spotted pictus. I started to read and read more on four-lines and learned they can grow several inches more compared to a spotted pictus. Good thing I do not mind keeping the four-lines. What if I had a smaller tank and that happened?
You cant even always trust you are getting the right fish and that it is named right. The kid doing the selling had no clue about fish it seems. I see this often. If the other two were working they would of known.
You will see many people doing things a different way and you will learn to take what you hear the most and apply it. That is what I have done and have an amazing tank!
Sorry this is so long, but its the truth and in the end you are the only one you can count on when you go to the store.
Stick with us and even if your pictus does not make it, lets get that tank cycled and get you where you want to be with happy pictus. As a pictus owner I can say they make me happy and are great fish to keep and provide me with a lot of joy and I would like to see you get there too.