I liked the art, and the ability to decorate, I went with the night theme and cherry blossom petals. With the random size and random rotation of placeable decoration pieces it took me a minute to realise that I needed to change the min and max size - maybe these controls should be closer to the random size toggle?
I wasn't sure if the fish were supposed to give me coins for keeping them in the pond, so I waited, maybe the fish should give you some every minute instead or if they are giving you it this way then it should be more?
I thought that the fish are cute and my favourites were black koi and the red and black koi. I noticed there is a typo in the spelling of "business" as it's currently the "bussiness" koi. I think it would be cool if some fish were labelled as "rare" or had special visual effects to give them some special feeling, even if they are from the cheapest gacha machine.