The tone of this conversation is becoming some what "heated", please refrain from singling out members with comments that are disrespectful.
Reading is an excellent way to start gaining knowledge but hands on experience is what counts more than anything. Think of it this way, I would never read a book on high voltage electrical supply installation and then feel I have enough confidence to work on it...chances are it would kill me. (I happen to be a qualified electrician - so can actually work on them...)
The knowledge gained is from seeing fish interactions, knowing how to stock appropriate to available local water parameters without messing with the tank chemistry too much. Trail and error to some degree...
In an emergency situation once, I was forced to keep a fully grown Oscar in a tank with some 6 month old fry...I had no option as the Oscar tank split a seam and needed an immediate reseal..expecting to see the tank empty of fry, I actually saw the Oscar cowering at the great attention 50 fry were giving it...5 days it took to properly reseal the tank, I did it too fast and had to start over. 5 days of a 13" Oscar in with tiny fry...NOT ONE SINGLE death of the fry. 3 years later, Oscar was still living with those same fish...I tried it and it worked. I would not recommend that to anyone but it worked for me.
Point being, you can read a ton of articles explaining that I should have had no fry left and one very happy Oscar with a large free meal...wasnt the case in real life. There are far too many variable to be able to offer wholehearted advice without the knowledge of the fish in question.
Cichlids for example, you can actually have 2 male Auratus in a small tank if the tank is decorated right and with a lot of line of sight breaks. 55g tank with 2 Male Auratus is like creating a mini war zone if the tank is not decorated well. It is not advisable to try this, but with giving a lot of line of sight breaks, ample hiding places and territory it can work.
When I first stocked my 180g tank, I followed a stock list identical to a moderator on another forum..it didnt work for me, there was a lot of aggression, the tank was identical in terms of stock list and only thing different was the way the tank was arranged with decorations. It was the close watching of the interactions of the fish and making changes sometimes quickly.
Take for example, would it be advisable when stocking an all male tank of African Haps/Peacocks to have another tank or several on standby to remove fish to in an emergency, how many fish would you keep in a removal tank and what would be the minimum size tank needed for this.