Koi Pond Problems

[juggernuts]

My stepdad has recently got a absolute bargain with some large koi, only problem is the pond is a little shallow atm which is to be rectified in the following months.

They are currently in a pond which is arond 16ft long by 5ft wide and around 3 1/2ft deep.

We are under no illusions for fish ranging 20" and around that its slightly shallow but as stated above this is a work in progress...

So to the problem, after testing ph - 7.8 nitrites and nitrates and ammonia all 0ppm as it is heavily filtered by a 300l 3 compartant filter box and 2 50l filter boxes fed by 3 seperate pumps. the koi seem very lazy as do the other couple of breeds of fish.the koi just seem to float about sometimes and some have frayed fins.

a couple have died and one seems to be on its way out as its currently floating upside down breathing slowly and has damaged fins.

There are no fish in there that I could imagine cause problems, there are only a couple of smaller orfes and 2 decent size goldfish.

Sorry if you struggle to make sense of this but trying to solve a problem that i cant clearly see is hard to describe.

Thanks very much for any advice or help!