I would do the following: 1. Set up the new tank, adding sand and whatever new decorations you have [if any], then fill it with water as you would for a water change in your current tank (including adding salt, etc). 2. Install all electrical things: heater, filter, etc. Use whatever media came with the new filter for now. 3. Leave this running for at least 24 hours.
When you are happy that the new setup is working as it should (at the right temperature, etc), you can do one of two things: a) move the existing filter from the old to the new tank, and run them in parallel. b) move the MEDIA from the old filter into the new filter. If needed, you can cut or gently squash the filter media to make it fit. Don't squash it too much, as that will probably prevent water from flowing through the media. This should be done at the same time as the fish are being moved across (of course, you probably want to put the fish in a bucket with a bit of "old tank" water and an airline dripping "new tank water", to acclimatise them, as the water chemistry will be slightly different between the tanks.
The "old tank water" will have very little of the "good bacteria" that you need for the filter. The good bacteria is on surfaces that are in the dark, mainly in the filter media, so this is what you need to move across.
I'm "cold starting" tanks all the time. Basically I use the same approach MatsP describes. I try to use about 1/3 old water and 2/3 treated tap water, add established sponge filters and stock the tank. As long as the filters already have been supporting a significant bio-load the new set ups are little different from just making a large water change and I do 75% water changes either once a week or every 4 days depending on the nature of my project.
If you have the patience you should at least get another filter running on the old tank and preferably "seed" it with some of the bio-media from the long established filter to speed up establishing a good level of bio-filtration. I recommend using at least two different filters on any tank so whenever one needs cleaning the other remains functioning. Then you also have an established filter to kick start a new tank. Many hobbyists keep sponge filters running in the sumps of their wet/dry filters so they have some ready to use filters whenever they need some for new set ups.
If you have substrate in an old tank of similar water chemistry you can take a few cups of substrate and stir it up in a small bucket with water from the established tank and decant the dirty water into the new tank. This will introduce a varied microbiota. Also this is one time when using one of the water conditioners containing hydrocellulose "artificial slime" makes sense. This will provide a temporary film on the new bio-media which will help facilitate the attachment of the nitrifying bacteria. This technique is widely used in commercial aquaculture.
also agree with those points above, but some more thoughts. If you are in no rush, I presume you have also a new filter. Connect this to your tank you have now. Wait 2 weeks and then set up your new tank and transfer the new filter accross. Then follow the other steps. I always have a second filter attached to one of my tanks, always great to have as an emergency back up. Good luck and hope you get a large tank for your colombian sharks and watch out for the spines. cheers jk
I am not perfect, but I say as I see it. Smile and enjoy Life
Running two filters in parallel for 2 weeks is probably not enough to get it fully cycled. And of course, you get the same effect by doing it the other way aorund: move the old filter to the new tank, and run both filters.