Fifteen years ago, Margaret and I built our fish pond in the back garden – during our recent trip to England, it failed, losing all it’s water – thankfully our house sitter, Matt, noticed the problem and got Country Gardens to visit and save the fish in a huge tank, which is what we found on our return to USA. All our fish (maybe 60 in total), and the lilies, were transferred to this 300 gallon container, with an air pump to keep the fish alive (we also needed to replace half the water regularly, since the hot summer sun slowly bakes the fish!) … the fish pond liner meanwhile had multiple failure points, still hard to conclude if the material simply perished and broke up at multiple points, or if some little creature attacked it:
We now had two choices – replace the liner ourselves, or bring in someone else; but swayed by a plan to improve the water filtering, we hired Mike / New England Water Gardens. One big hassle with our original pond was needing to pull out and clean the pump / filter bucket twice a week; Mike planned to install a lower maintenance skimmer, and also contruct an improved waterfall, while also of course replacing the liner. The new skimmer / waterfall also includes a bio-filter tank at the top of the waterfall, so there’s no “T” splitter any more for the pump water, instead Mike used a big 2″ pipe, for higher flow at the waterfall!
Once the basic construction and pipe installation was completed, Mike began transferring water AND FISH from the tank to the new pond:
Then while the pond continued to fill, Mike finished the waterfall construction; meanwhile the fish, confused in their new spacious pond, stayed as hidden as they could:
Finally all was finished, the pump started – and we had an impressive and big new waterfall, and today, the fish all seem settled as if nothing had ever happened!
M&I / GBMINI 
Pond looks great. Pity you cannot make up your mind when it was built. You have said 12 and 15 years. It would be 14 if built in 1998, but hey.
1998 … 15 is rounded to a nice number!