@ the red devil comment.
You know, i've seen large red devils and FH in tanks that are 55g... i've also seen them in tanks that are much smaller...
Really... its hard to say what a minimum tank size is... for me Minimum size is EXACTLY that, the BARE minimum... I want my fish to have as much room as i can give them, a single Flowerhorn could be housed in a 100g for me, the foot print on the 100g is a lot different then a 75, etc etc...
People say things like, Minimum size is a 75.. Well a standard 75 is the same foot print as a 90g, the 90 is just taller, will he use that space well ? Probably not as well as he could... Longer wider tanks > Taller tanks...
Really as i've come to learn it ends up like this... Think of all the fish that have bad homes... WAY too small, stunted fish, bad food bad water quality, etc etc...
I honestly feel if you can give a fish a better home then it has, even if the tank is too small, go for it... You can always upgrade someday.
What really got me thinking about this was a discussion here... A guy is having a 4000g tank made... sounds big right ? Well, he wanted an Arapaima for life in there. Nearly everybody agreed. Not big enough.
But one guy here thinks like me... he said "Think about it... Most pima dont have good homes at all... 4000g is better then a lot of them end up in..."
Makes you kinda think huh ? I think if you can give the fish a happy home and you CAN someday upgrade a fish can do well in a tank thats a bit too small, just needs a little extra love.
BTW, here's another quote i've seen here, this is advice given from some girl at petco to an MFK'er:
MFK: Hey, i see on this card Green Terrors only get 5" long? This isn't true, I have one at home in a 180 gallon community tank and he's easily 10"
Petco: Ohh, thats the problem... His tank is too big, put him in a 20 gallon when there small and they will reach their optimal size.