3)) The electrics. My Dad fitted an RCD consumer unit as its in the garage and there will be a lot of water floating about in there. HOwever, issue a) RCD doesn't work with florescent lighting, so it keeps tripping! The sockets also trip....so there is an issue with one of them - my Dad is on the case!
This is quite a major issue you ought to get sorted as you don't want to return from a couple of days away and find that the moment you left the RCD tripped.
If I were doing it I would put in 2 feeds to the fish house, one for power (ie 13A sockets) and one for lighting, oh and do it properly with buried armoured cable (18 inches down) 2.5mm twin and earth, even in a plastic pipe doesn't last too long before you will have issues, usually broken core due to thermal effects as well as UV light degrading issues.
You would add another small consumer unit (say 4 way) next to your exising consumer unit (connect with 6mm cables to main house isolator) and fit a 32A breaker (not RCD) for power and 16A (not RCD) for lighting. These breakers protect against cable and other gross faults. You then wire to fish house with 4mm armoured cable for power and 1.5mm for lighting. In fish house you have a consumer unit for power say 4 way and another for lighting. In lighting one you put a single RCBO to protect lighting. In power one you fit multiple 16A RCBO's, say one for tank power, one for other fish power (water pumps, etc stuff not necessary 24/7) and other for user accesable 13A sockets eg kettle etc.
The reason for doing this is to limit the scope of failure during fault conditions and allow isolation of different areas.
The seperate light circuit is obvious, get an issue with electics and it is a lot easier to fault find when lights work.
Splitting the load in the fish house is sensible to allow sections to be powered off, for when updating/modifying, but mainly so you won't loose power to your tanks, when you plug in a wet power tool and trip the RCD.
All this comes as "voice of experience" in wiring power to workshop at end of garden. I initially wired 4mm armoured cable, but had numerous issues. Basically should have been 6mm for the length needed, as sometimes when I turned the lathe on the power dipped tripping the MCB plunging me into darkness. Also damp often caused the RCDs to trip when first turned on thus back in darkness again !!. Replaced cable (running down garden at base of fence) with 6mm for power and 1.5mm for lights and tripping RCD's now wasn't such an issue and a lot easier to fault find when the lights were on.