Don't install bottom drains.

Why? 2 main reasons:

1. You have to cut a hole in the bottom of the rubber liner to fit the
drain pipe. No matter how well you plan this and use the top of the
line sealants, it will leak. Maybe not in the first 2-3 years, but no
matter the claims of sealants, they expand and shrink as the
temperature changes and do not change exactly at the same rate as the
rubber liner. Over time this connection fails. If you give an extra
tightening turn or two to make the seal tighter you are actually
pushing the sealant completely out of the space and make the lifespan
of the seal weaker. Once you get a leak in the bottom of the pond then
there is a whole list of bad things that can happen. Why cut a hole in
a perfectly good liner that will last for 25 years or more?

2. The holes in the bottom drain cover plate get clogged up with leaves
and debris. Let's say that the wind blows a plastic bag into your pond
and then the bag eventually falls to the bottom and obstructs the
drain. Now your water pump is sucking into a vacuum and the pump will
burn out. You can avoid this by installing 2 bottom drains to divert
any negative pressure from creating a vacuum, but now you have 2
potential places for a leak. Also God forbid, but let's say a child is
wading in the pond and slips into the deep end where the drain is. If
his bare leg comes up to the drain it can create a very tight holding
force and actually prevent him from coming up for air. The suction
pressure is so great that even a strong man would need to struggle to
break loose.

We have found that for residential installations the Savio skimmer
http://www.pondkoi.com/pond_products_pond_skimmers.htm works best. As a
matter of fact we won't even offer anything other than that for sale on
our website because we don't want anyone to use inferior products. We
have tested virtually every skimmer on the market and we are abosultely
convinced that the Savio is the best unit available. It does not need a
bottom drain.

You can visit your pond once a week and scoop out anything that has
settled to the bottom and be done with it in 5 minutes or less.