I inhereted a Koi pool when I bought my house. The pool was installed
with a presurised sand (swimming pool) filter. There was so much leaky
plumbing, and it seemed that when I got one thing fixed another
problem arose costing a lot of money.

Despite this the fish were getting prone to infections, and I bought
two large biological filters. Installation took about half an hour,
just had to redirect the pump output, and they are so easy to work
with and maintain.

I think a sand filter is ok if you have already got a biological
system, but on thier own I was less than impressed!! My pool is 1800
Gallons, but I have installed a biological filter which contains 20
gallons of medium and which could handle a pool twice this size!!

I am not sorry I invested the money, and now enjoy a fairly care free
system!!