if you have a 20 gallon tank, upgrading to a 29 gallon isnt much difference, i would recommend a 55 gallon, if you just spend a little more bit more money youll be far more satisfied. that would be my suggestion, but if you insist on switching from those tanks, i would say the first thing you need to do is take the fish out and put them in a bucket (with water). second take all your gravel out using a plastic disposable cup or something of the sort and out the gravel into your new tank. next syphon the water from your old tank into the new tank, it will help cycle the tank, then you need to setup your filters/heaters, and then turn on the filter/heater, and then add your fish, simple as that..but i would strongly recommen a 55 gallon, i first had a 10 gallon , and decided that wasnt big enough, so then i upgraded to a 20 gallon, and that wasnt good enough so then i bought a 55 gallon and i am satisified. youll save yourself money if you get a big tank this time around, trust me..