A LOT of mistakes can be avoided by using the Internet and doing searches. As to fish I would look here first for followups as I have grown to respect the advice given but only after doing some initial research on my own.
Still I also have a cockatiel that, at times, I try to get to eat fresh stuff instead of just his seed mix.. Sigh, so far, the only success I've had has been shredded carrots and mango but the point is that I was thinking about trying avocado. Fortunately I researched on-line first and found that the stuff is VERY toxic to the bird. If I had just tried the avocado without doing the research my bird from Hades would most likely be dead if he had eaten even a small amount. It is funny in a way as to what things will and won't work with different animals. Another toxin for my bird is apples. The meat of the apple is fine but the seeds are quite toxic.
While most of the above is dealing with research as to my bird it also applies to aquariums. For instance I would never introduce a new fish without researching the needed water parameters to make sure my tank is OK for the new fish and if it would get along with my current fish. Sure I said that I respect advice here and could just ask if a fish will be OK but I find it more rewarding to do my own research and, if I'm not sure on something, then ask here. I just find it more satisfying to do my own research initially instead of just falling back and relying on others without even making an attempt myself. Also I tend to find it normal that when doing research on one topic I end up learning a lot of side things that I didn't even know were questions until I saw the info.