Hi,
There is another way to do it.
1) a ,45 kw pump. An old one that has been reconditioned will do just fine.
2) flexible piping.
3) 40mm poly piping to waste
4) aluminium pole to support piping.
You simply suck through the open pipe to waste. The water you "loose" is not lost as you need to do regular water changes anyway.
It really depends on what you want to vacuum for. If it is leaves or a lot of sediment, then I would go the above route with a larger leaf skimmer attached to the fromt of the pump.
If its just fish waste on the bottom - what happened to the bottom drains?
If its show tanks and you just need to keep the bottom clean from time to time - I would go via the sand filter with fine sand. You return the same water to the ponds after removing the fine solids. Also you would polish the water before returning it to the ponds. This adds to the clarity of the water and beauty of the koi.
The vacuum pictured in the previous posts will work just fine. Also look at WAP products they may have something that can be used and Kartcher. These companies have wet/dry vaccum cleaners designed to such up water.
Protein Skimmers
You can also build one of these yourself if you want. Built one with Neville Marias back in about 1991. Used 110mm piping some fittings and a plastic venturi. Blasted - sorry - pushed water through it with a swimming pool pump - and yes a valve to control the inflow of water.
On fresh water ponds you cannot really get a lot of foam. Nothing near what you get in marine tanks or in trout farms (massive stocking densities).
You will get a more foam as the stocking densities increase or if you change the water quality such as when you add salt and the next day there is foam on the pond. Cheapest way is to have an overflow and just top up the pond till it overfow a bit in the mornings when the foam is worst.
I must honestly say I doubt if protein skimmers are worth it on the average koi pond. Rather increase your water changes to get rid of DOC (dissolved organic carbons) etc.
Hope this helps.
Chris