The biggest factor to cover first is the insulation, heating is the biggest cost when running multiple tanks so the less heat the room loses the better it is for your bank balance.
The best insulation to use is foil backed expanded polyurethane (spelling?) board (kingspan or celotex) which can be screwed directly to the walls using screws with washers on and rawl plugs, unfortunately its quite costly at about £20 a board so isnt an option for anyone on a tight budget.
Polystyrene sheet is another option but it tends to trap moisture which can lead to big condensation problems and is especially not recommended if the garage is integral to the house due to possible problems with damp.
Wicks do a foil backed bubble wrap on a roll which is designed to insulate under floors and ceilings but can be stapled to the walls and ceilings with a heavy duty stapler, it does a reasonable job (obviously not as good as the boarding) and is more affordable than the boarding but does make your fishroom look a bit like a set out of star trek.
Do not use rockwool or fiberglass type insulation as it just soaks up moisture and becomes a soggy useless mess.
Ventilation is important too so if the room isnt vented then you will need to fit an extractor fan on a timer or a couple of vents just above floor level (heat rises so carries the fresh air up with it) to let fresh air into the room.
Next you have to work out how much heat you will need, tanks need on average 1 watt per litre of water so you need to add up the litres and see how much wattage you are going to need, once you get to 1000 litres/watts or more you are better off heating the room rather than the individual tanks, oil filled electric radiators are good and fairly cost effective to run, especially if you set them on a timer to have extended off periods over night, it is also a good idea to run a spur off the central heating system to a radiator with a thermostatic valve in the fishroom so that when you are heating the rest of the house the fishroom gets the benefit too.
If possible plumb a water source in the fishroom and if you can set up a drain system too then its even better, lugging buckets of water in and out of the room and running hosepipes through the house is a pain and will get you grief from the mrs.
Make sure you leave space for storing things like buckets and all the other crap we tend to collect for our hobby, its suprising how much room things like this needs and there is nothing worse than having a clutered fishroom where you constantly trip over and have to move things around when you are in there.