With regard to fitting an RCD to the supply to a pond,
I would fit the RCD in the house as has been suggested so that it protects everything outside the house.

HOWEVER,

I believe the standard RCD's available in the UK as rated at 30 and 50 mA trips, I assume you are UK based since you use the term RCD and the email address. After seeing, and being involved in, some interesting debates on electrical safety on some of the american boards, where earthing of the pond water directly to the ground and unearthed devices are not uncommon, I performed a series of experiments to check the effectivness of a 30mA RCD

http://www.fishcrazy.co.uk/forums/in...4593#msg124593

http://207.228.252.5/forums/showthre...ctrical+safety

You may have to register with either or both of those boards to see the threads, sorry.

DO NOT attempt the experiment yourself, I had a reasonable idea of what I was doing

The conclusion of the experiment is, that if a submerged 'break' in the insulation of the live etc occurs some distance away from a good earth a 30mA RCD may not 'see' enough leakage current to trip and thus the water will still be live. BTW my 30 mA RCD was and is working perfectly well, in fact I measured the current required to trip it and that current was, from memory, in the low to mid 20 mA range, ie below 30mA.
An american, Johns, repeated the experiment with a US spec 5mA tripping RCD/GFI and that did trip, see the 2nd of the links above. I would therefore suggest that you try to get a 5mA trip RCD, unfortunately I dont know if they are available.