Yes, it’s extremely easy to make mistakes, mostly due to the abundance of misinformation about fish keeping online (that goes for other exotic pets as well actually). And a lot of the time, even when you have all the correct information, theres just things you learn from experience.
Im someone who definitely isnt an expert by any means, but I think I’m a decent fish owner, and some mistakes ive made in the past few months are:
-leaving the lid off the tank and having a fish jump out and get injured from falling
-having a fish die because it stayed in the back of the tank a lot and I didnt check on its physical condition and assumed it was healthy and fine
-ridiculously overestimating how much food
guppies needed and killing two
Mistakes are easy to make, and hopefully teach you things, but like the person above me said sometimes you never even find out what happened.
If you would like help figuring out what is wrong could you give us your tank info, such as tank size, the fish youve stocked, what you fed them, their symptoms as they died, if the tank is cycled, the parameters of the tank, etc. please?
