There is much to know about keeping fish. The great thing is you don't have to know everything to do it and can learn. Oh, and even when you know a lot you can learn more, and also still make mistakes.
Yes we have all made mistakes, been uninformed, misinformed, under the wrong understanding and so on, lol. But you now have a great army of resource here at AA. And things should get better.
Once you have a good healthy amount of BB built up, never replace your filter pad until it is falling apart. Clean it regularly, gently only with treated water to keep the bb safe, and QT new fish, even if you have to use a Plastic tub or 5G bucket as a tank.
Once I stopped changing my filter pad and rinsing with tap water, most of my problems went away!!!
each week, too. Don't over feed. Don't overstock, keep compatible fish with the same tank requirements together. And use a high quality food. (I like Hikari as a basic food, doesn't cloud the water) You will have eliminated a large amount of your troubles. Hope I didn't leave any out, but I am tired. Oh, and get a API Master test kit, online they are reasonable (if you don't already have one)
Happy fish keeping and