1. how much do koi grow a year?? like, how old is a 3 in. koi, & how old is
a 12 in.?? seems like my goldfish are growing & the don't.

He... for me I would say the 3 in. is a 1 year old and so is the 12
in. (at least for a couple of my fish)

2. also,,, I have read on here about how koi love to trash plants. is there
a certain size they get to THEN they start doing this?? and/or do you need a
gang of them?

Lillies / lotus in reasonable numbers survive but for other things
(underwater) you need to have a lot of plants. I recommend separate
koi pond from plants.