Wow a whole forum dedicated to the what we've done wrong. Geee where to start. 
Can I just make a list of all the things I've learned no to do over they year from my own mistakes and those of my friends? Well here it goes.
Do not keep Oscars in a topless tank- no matter how many times they jump out and flop around on the carpet, they do not learn.-me
Do not feed large bullheads to a tank of red bellied piranha- Talk about a MESS to clean up- friend
Do not grab bag with hand when bagging Black Piranhas- unless you like to watch yourself bleed- kid working at lfs
Do not believe the fls when they tell you a Scat is fresh water- luckily I found out it was brackish and kept the tank well salted- me
Do not buy a tiger muskie if you only have a 55 gallon tank- me
Do not put 4 dozen feeder in tank with said tiger muskie at once- TO MUCH BIO LOAD- me
Do not put blue gill sun fish in a tank with other N. Am natives like crappies and walleye- the sunnies are terrible bullies- friend and me
Do not buy a silver arrowana if you only have a 125 tank- friend
Do not keep red bellied piranhas and Oscars in the same tank and forget to feed them- friend
I'm sure there are more, but this is a good start.
Can I just make a list of all the things I've learned no to do over they year from my own mistakes and those of my friends? Well here it goes. Do not keep Oscars in a topless tank- no matter how many times they jump out and flop around on the carpet, they do not learn.-me
Do not feed large bullheads to a tank of red bellied piranha- Talk about a MESS to clean up- friend
Do not grab bag with hand when bagging Black Piranhas- unless you like to watch yourself bleed- kid working at lfs
Do not believe the fls when they tell you a Scat is fresh water- luckily I found out it was brackish and kept the tank well salted- me
Do not buy a tiger muskie if you only have a 55 gallon tank- me
Do not put 4 dozen feeder in tank with said tiger muskie at once- TO MUCH BIO LOAD- me
Do not put blue gill sun fish in a tank with other N. Am natives like crappies and walleye- the sunnies are terrible bullies- friend and me
Do not buy a silver arrowana if you only have a 125 tank- friend
Do not keep red bellied piranhas and Oscars in the same tank and forget to feed them- friend
I'm sure there are more, but this is a good start.
