<a href=showthread.php?s=&postid=11430752#post11430752 target=_blank>Originally posted</a> by Rhodophyta
That sure makes me think the pond liner approach makes the most sense in a retrofit for all the above reasons. You don't remove any water or stand braces. You can shove part of it out of the way to work on something else. You can change your mind later about how tall it is, or how much floor space it takes up if a skimmer cup is inconvenient, or you need floor space for some new equipment.
To get the liner into a corner, I prefer to fold it in flat on the bottom and walls, which leaves a triangular tab inside each corner. You can tuck the tabs behind, fold them over the front, or roll them. To avoid sharp creases in the liner which might cause a leak (fixable) some day, you can round out the corners before installing the liner with some modeling clay or even small bits of crumpled newspaper. If you don't care for the unfinished look, keep the doors closed, :lol: :lol: or you can cut the tabs off and seal the cut with pond tape or trim the tabs and glue them with pond repair glue. But then you are committed to that shape and size unless you pick up a new scrap of pond liner. Often there are scraps at a pond store that you can get discounted.
If you use the stand walls for support, you can work around obstacles such as vertical supports and get the maximum volume in the sump if you want that.