some goood suggestions there especially the water meter. If I were doing it I would try to save and reuse as much of the existing water as I could and as has been suggested those paddling pools are very useful. If you decide to try it get the ones with the inflatable collars or frames and plain walls. I found the ones with inflatable walls unreliable, Also if the pool in full sunlight try to cover it, they get warm and may be very bright. I had a temporary tank with a clear plastic liner over white polystyrene sheet over wood and I didnt even get it filled (in bright sunlight) before my eyes were hurting I emptied it and added a layer of black plastic to cut out the reflection from the polystyrene. I dont know what the brightness would have done to a fishes eyes over a prolonged period. If the foot print of you new pond will be about the same as the existing one, ie 10' x 6' I'd suggest you dont try shelves but go straight down to depth, it ups the water volume and makes lining easier. Plant shelves can be rested in milk crates or hung off the sides. Such 'suspended' shelves also offer cover for the fish. I would 2nd 3ft or deeper, my goldfish pond is 4ft.
A word of warning about sticklebacks, especially those taken from the wild, they can be disease carriers. They can also carry an intestinal worm that causes them to swim on the surface to attract fish eating bird, the worm completes the next stage of its life cycle in the bird.
Personally I wouldn't put stickle backs in a pond with ornamental fish but I think they are great in a wild life pond
If the fish would be from a tank then, as suggested, it is a bit late in the year to be adding fish, if they are from another outdoor pond etc then I would leave it a month or so unless you can add appreciable quantities of matured pond water. The micro organisms are also probably slowing down at this time of year too so new water would probably take quite a while to 'mature'.
Re the liner, if you can, fold and tuck away the flaps rather than trim them until you are certain that you like the final shape, even then I wouldn't trim them if that isn't a requirement of your building.
Re the brick wall, rather than cement bricks together, which may not be as strong as you think, note the "may" I dont know for certain, I have an alternate suggestion. Dig out one spade's width and depth or slightly more of the upper wall and pour a concrete collar in place of the soil. I had to do this around what is now my lily pond because the soil wasnt as stable and load baring as I thought it would be. If you want to do this all in one pour then you would need to shutter the pond side and shutter it well as wet concrete will deform poor shuttering. Two layers of 1/8", 3mm, hardboard with good staking might be sufficient yet flexible enough for you to form the curves you need. If you have your own mixer and can pour the concrete over several days then another way to do this would be dig out the soil behind the bricks and carefully wash and scrub the soil off the back of the cement already in place. Clean up the trench, I use a workshop wet and dry vacuum, a £30 B&Q offer, and then line the floor and outside wall with thin plastic sheet but leave the back of the cememt exposed. Then pour a couple of inches of concrete into the trench and gently tanp that, cover that with the plastic sheeting to keep it damp and let it set overnight. The following day uncover the concrete and gently scrub off the surface of the 'old' concrete and wash it, again I'd vac the water out. Add another couple of inches of concrete, gently tamp and recover etc etc. when the trench is full keep the concrete damp and covered for at least 3 days. The plastic lining the trench helps keep the concrete clean and, whilst setting, damp. After 3 days the surplus plastic can be trimmed off. The concrete will bond to the cement and bricks so they become part of the collar. I did the latter around one of the above lily pond but my 'bricks' were flagstones, the edge is now rock solid.
Berm!!!!!!!!!!!!!! please!!!!!!!!!!!!!! levee....... or how ever it is spelt, this is an UK board not an American, btw I am taking the mickey.
Re carpet etc I would suggest synthetic and it is probably more rot resistant but I have a word of warning over combination of depth and the contouring, I suspect that with the combination you are going to end up with some significant folds in the liner. Take you time filling the pond and work the folds as it fills, personally I would consider having some temporary water storage facility handy so that if you decide to shift the liner about you dont have to dump the water. The circular paddling pools with inflatable collars sold by the likes of B&Q make good and fairly cheap holding tanks. I took me a week to fiddle with my liner but my shape was worse than yours.
Re underlay, I used sheet polystyrene in my fishpond but that is inside a concrete shell. I used sand on the wildlife pond and the plant pond, it can be used, damp, to line sloping walls but vertical walls would be a bit much. Polystyrene is a BAD idea for floors on which you will want to walk, it crushes under the concentrated load generated by feet and takes a set, sand feels hard when there is a good depth of water above it but it still has some give. If you use sand on floors walk it in bare feet, you'll soon find the sharp points you missed, likewise if you try damp sand on walls use bare hands to pat it into place. If you use carpet make sure it's tack free!
If those/that are/is your fish, IMO that is abnormal. My 'pregnant' females would be neither as deep nor as wide and rounded as that, they would be fatter than normal but not by much. That aside did you have fish in the pond, if so you should have quarantined the new fish? Spawning this late wouldnt be that odd though it is probably near the end of the season.
No comments about what Hozelok said other than about the sponges, I didnt realise your on fine grade foam.
Re lift, actual physical lift is NOT measured from the pump, it IS measured from the water surface above the pump. Gravity deals with the lift from the pump to the water surface so the pump doesnt have to
The titans are dead simple to strip and clean, no tools are required, if its the old version be careful handling the cases as theyu can seperate and allow the pump to drop, the new titans have twist catches holding the case halves together so they cant accidently seperate. Describe how you clean the media.
Well I did/do have a ecoxxx or whatever it was called a few years ago on my fish pond, it was/is the smallest in the range and it does its job but I did find the spray bar restrictive a made my own. However I have made a bigger DIY one since then. For me the advantage of the gravity type is that when the media clogs the flow bypasses the media and the flowrate is maintained BUT make sure you DO NOT retrict the output because the filter will overflow and water will be lost
Hozelock produce a range of filters with names starting "eco" http://www.hozelock.com/aquatics/I053.asp?region=ROW. I would suspect upping the size of the filter will reduce the cleaning frequency and I dont see how a filter can be oversized unless thee is a minimum population density for the bacteria involved