How big is the tank? If its big enough you can leave everything in there. If its small you might need to take the fish out so you dont suck him up, even though they still might try to swim in in the big tanks for somereason they think their poo leavingistheir food coming
. As for the decor, it is fine to leave it all in there but you will probably knock it all around and dislodge the plants anyway. Every month or so I like to remove it all so I get a clear shot at the gravel so I can suck up every bit.