not all piranhas are timid...sometimes you get an aggressive fish even at juvi stages. all my piranha tanks don't have hiding spots. I keep it simple...all natural sand type gravel with a big driftwood and natural light (not direct sunlight). if you provide hiding spots like lots of plants they will hide and if you keep the tank simple and lots of swimming space they tend to be more active. you don't go into keeping piranhas bec you think they are killers...you will be disappointed. they all have different personalities...I just love them all.