Which part of Australia are you in?
Use 4 inch thick polystyrene foam sheets to insulate the walls, door and roof. Seal up the windows too if the shed has a window.
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If the shed is big enough, remove one or more of the roofing panels and replace them with clear Laserlite sheeting. Double glaze it too if you can. Have the 2 panels about 5-6mm apart.
Alternate the roofing panels so you have 1 clear, 1 tin, 1 clear, 1 tin, etc.
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Get some rubber floor mats from Clark Rubber or somewhere like that. They are made from recycled car tyres and come in 1/2 inch and 1 inch thick mats that are 1 meter square. Get the 1 inch thick mats.
Put the mats down and leave them for 24 hours before cutting them to fit. They squish out when stacked on top of each other and shrink back to their normal size when laid out individually.
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If the shed is not on a concrete slab, pour a 4-6 inch thick concrete slab for the shed to sit on.
If you have to do any concrete work, do it when it's cold so the concrete has time to cure and doesn't fall apart. If you pour concrete or build a brick and concrete wall in hot weather (above 25C), you should water the ground before you pour the concrete, and then water the concrete with a sprinkler after it has been poured. You keep the concrete damp for 24 hours after pouring so it has time to cure properly.
When concrete doesn't get enough time to cure, it becomes brittle and cracks and breaks up very quickly. You see this in houses that have been built in summer where the concrete driveway has cracks in it, and where walls get cracks in. These places have been built in hot weather and the concrete dries too quickly and becomes crap.
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Get an electrician to run a couple of double power points to the shed and have circuit breakers built in.
Have a single 4 foot long fluorescent light unit fitted to the roof, or a couple of compact fluoros or LEDs. This light can be used in the early morning or evening/ night if you are working on the tanks.
You can have lights on the individual tanks if you like but there is usually enough light coming through skylights (clear panels) in the roof.
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Get a large blower type air pump to run undergravel and air operated sponge filters.
Get a heater for cold months if you live in the southern half of the country. Gas heaters are cheaper to run but you need to have a flue for them otherwise the fish suffer from carbon monoxide poisoning and you get lots of deformed fry.
Electric heaters are expensive to run but don't produce harmful gasses. If you have solar power then go electric heaters. One decent sized heater will warm the entire fish room and if the room is well insulated, then modern electric heaters are quite efficient.
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Build stands that are 2, 3 or 4 tiers high. The more tanks you have, the more water to hold the heat and the more stable the air temperature. Plus the more tanks you have, the more fish you can keep in the room.
Have 8-12 inch gaps above each tank so you can get in them to work on them.
The bottom tanks should be 6 inches off the ground so you can syphon water out of them. The gap under the stand also allows you to see if anything is living under the stands (rodents, spiders, dead fish, etc).
Have tanks that are 12-18 inches high.
Maybe have a couple of water holding containers, depending on what your water supply is like.
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If you fill the tanks with a garden hose, turn the tap on and water the lawn or garden for 5 minutes before using the hose to fill the tanks.
Garden hoses have a softening agent in them to stop them kinking, but the softening agent is toxic to all life forms including fish. Most softening agent leaches into the water in hot weather and running water through the hose for a couple of minutes removes any harmful chemicals that have leached into the water, and flushes out any copper from the water pipes.
Black poly pipe (irrigation tube) does not have softening agents and is safer to use. Again though it's a good idea to run water through any hose for a few minutes before filling tanks with that hose.