Best way to changeover tanks?

I'd be getting the new tank ready.

Clean it well and get it situated. Your best bet is to do everything in the space of one day.
On moving day siphon as much water out as you have room for- rubbermaid garbage cans hold 30 gallons and cost about $5.95 US, so two of them would hold all your water.

I'd put all the fish in one garbage can of tankwater (with an airstone if you think it'll take more than a couple of hours). Then place the filter parts and any decorations/plants in the second garbage can of tankwater.

You can now start moving some (or all) of the old substrate into the new tank.
You could rinse the old substrate a bit in dechlorinated water but don't use tapwater or else you'll kill your biobugs. Any rinsing would be easy to do while it is still in the old tank- no fish or decor in the way. Just remember to use dechlorinated water to rinse it- it doesn't have to be tank water, just dechlorinated. You don't want it too clean- just make sure it's relatively free of chunky gunk. Some gunk is good- it's called mulm and contains biobugs that you want to keep.

You're basically going to move everything into the new tank- the old substrate (or as much of it as possible), then the garbage can full of old water, the old decor/plants, set up the old filter that is already cycled, then lastly the garbage can of water and the fish inside it.
As long as you use dechlorinated water to rinse stuff you won't lose your biobugs.

If the new tanks needs a completely different substrate you could always put some of the old in a mesh bag and set it inside the new tank for a while. Used substrate is such a fine source of biobugs that it would be a shame to toss it out though.

How long will that take to heat up if I'm using 2, 300w heaters or is that a bit of overkill?

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Depends on what temperature water you start with and the average ambient temperature in the room. For initial set up I'd just use a bit of hot tapwater to adjust the temperature.

Flohrie - Personally, when I am moving tanks and such, I'll set the filter up to run attached to a 5 gallon bucket that I use for waterchanges. Just siphon some of the tank water into the bucket, hook up the filter and you're good to go for a long while. That way you can take more time to set things up the way you want without hassle.

I also would not bother with rinsing the old substrate, all of those nasties and mulm is good at keeping a tank stable and healthy. Store the gravel in a couple buckets. Just allow the filter to cycle the water through for an hour or so to get the debris out of the water column once the water is up to tempature before adding fish. Also, wait until the tank tempature is correct before adding your substrate back into the aquaria.

600w on a 120g is fine, espescially since they are seperate heaters and if one gets stuck on it shouldn't raise the temps much more than 80-85F by itself. As Cearbhaill stated, you're better off adding a couple buckets of hot water to the colder tap water you are probably using to fill the tank. Even with 5w/gallon for heat, it could still take a few hours to get up to tempature if the water is somewhat cold or ambient tempatures are low.

Regards,
Raithan O. Ellis

I suggest you get the new tanking running asap and fishless cycle it while you have the time.

Originally posted by Flohrie
Hmmm but my only decent filter is on the current tank

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The problem of cycling is eliminated if you reuse your old unwashed substrate and don't clean the filter between the swap. Keep the filter running on a bucket, or keep the media in a bucket of water for that period of the move. You shouldn't have to worry about a thing.

Regards,
Raithan O. Ellis