Pond upgrade.

I have only been keeping Koi for about a year. The decisions regarding my pond and filter system were all based on information from commercial garden centres. Since then I have been introduced to other Koi keepers, Koi Carp magazine and sites like yours and learned the error of my ways. I often visit your web site as it contains such useful information and advice. It is a great site.
Currently my pond is 13.86 ft L, 6.69 ft W, 5.5ft D. The top 21 ins consists of 15 ins wall sitting on a 6 ins thick 8 ins wide concrete base. The pond is constructed using a liner. The ground is all clay apart from the top 9 - 12 ins. I am now looking to upgrade my pond.
The upgrade is going to be increase in the height of the wall to 21 ins., removal of shelf, introduction of gravity feed drain and a skimmer. I will be laying a concrete base with a 6ins slope from all sides to the drain. I do not wish to reduce the internal dimensions of the pond.
Whilst I could take the wall down and break up the foundation I really do not wish to go down that path, unless all the other options prove to be dangerous, unworkable or not sustainable over a long period of time.
My options are: -

1) Install a box liner.
2) Fiberglass my preferred option I would get this done professionally, but the preparation would be one of two options.
I would be grateful for any comment.

Remove 2 - 2.5 ins of clay from underneath the concrete foundation, this would leave 5.5 - 6 ins sitting on clay whilst this part of the upgrade takes place. Line the exposed clay with my existing liner so creating a waterproof membrane.
Then either build
a). 2 ins thick concrete wall supported by rods within the concrete and possibly Fibromix.
b) build a wall using 2 ins thick solid blocks.
Skimming the sides (including the concrete collar and wall above) and base with concrete and Fibromix.
All the concrete mixes used would contain waterproofer.
I have already upgraded my filter system to a R.T.F. 3 bay system.

In case it is of interest I can send a copy of the details of the upgrade I made for Richard Wood who hopefully will be undertaking the fiberglassing. He has given me a quote of £750 for the complete job including, travelling and VAT costs.
Mick - UK

Dear Mick,
The problems you are having with building your first pond, and then finding out what is required for a proper koi pond is something most of us have previously encountered, and many more will also encounter in the future.

If you look at the problems when building Mark's pond you will see how he overcome it, although his initial problem was a high water table. This is OK if you are using a liner, but not very suitable if you are going to fibreglass it.
You could follow the same sort of fix as he has done, but as you are thinking of getting it fibreglassed then I personally would go the whole way and remove the 15ins wall and break up the foundation. To ensure that the walls are strong and will last for many years then I would consider that you really need 9" walls. This is important as clay sub-soil, which is what we also have in South Wales, is notorious for subsidence if we have very dry periods. (Not very likely this year).
The blocks are then rendered as described on KOI CYMRU website.
The concrete base should be at least six inch deep with steel mesh reinforcing. The base should be benched towards the bottom drain so all the debris flows that way, and it should be nine inches. Anything less than this will not be sufficient as any blanketweed growing on the base will tend to stop the droppings from moving.
You do not mention bottom drains, but I do consider them essential, and you will later regret that you did not fit them.
If you did decide to move the existing wall then is it possible to make the pond a little wider than the 6.69ft? This is just some first ideas. Let me know what you are then deciding to do and we can discuss it further whichever way you want to go.
I would be very interested to have any details of the upgrade, and will add it to this letter. Perhaps you could take photos of the work as it is progressing.
I am always looking to add new material to the website to help others decide what they can do with their ponds, but very little is coming through.