beautifl fish = 綺麗な魚(きれい な さかな) kirei na sakana
kireinasakana
The second one "Kireii Sakana" doesnt make sense; however, if some one says them separately "Kirei (beautiful)" and "sakana (fish)", it could be ok.....
For instance, "hey look! fish!! beautiful!" or "ne mite! kirei! sakana!!"