How often did you change the water? 1 gallon tanks and smaller require frequent water changes.
It could also be that as (I'm assuming from your description) he didn't have any plants/decor, he was feeling insecure or bored. Also, be careful that the plastic plant isn't sharp enough to tear his fins!
Do you treat the water? What with?
Puck (Yellow koi HM king) / Scar (White HM) / Salt 'n Pepper (Silver HMPK) / 'That fish' (Copper butterfly HM) / No name (White DT) / No name 2 (Turquoise/white marble HM) / No name 3 (Lilac HM)
Bo Peep (Cambodian VT) / Goldilocks (Yellow VT) / Joie de Vivre (Multicolour VT) / Lil' Peep (Blue VT) / No name (Dalmatian VT) / No name 2