generally, I would agree with delgado, because its pretty much the only surefire way, and rottbo's idea I think would also work, although I don't use powerheads in any home tanks, so i don't really know. but depending on how much time you have , you can set up an oxygen farm. Any tank 5+ gallons will do it. use some high quality substrate in the bottom of this tank, and fill it with tank change water, no filter necessary. then, put in as many cheap plants (java moss, jave ferm, etc as you can find. then, seal off both tanks with plastic, but first put a cheap air pump w/hose from one tank to the other, and seal it up completely, sealing the holes w/ caulk or hot glue. A nother length of air tubing should go froma Co2 farm to the o2 farm. a co2 farm is basically a soda bottle w/ sugar and yeast. This step (co2 farm) is optional. Be sure to cover the filter on the big tank as well. I cannot offer a guarantee on this method, because you are basically turning your tank into a microcosm for a short period of time, and it generally takes a couple days of adjusting the two tanks and testing the system to get it right. The more time you spend on it, the better chance of it working, because you have to fit the need sof your fish. Really, the best choice is to do as delgado said and just move the fish, probably the whole setup too. If you don't move the stuff, EVERYTHING MUST BE COVERED. I only use my oxygen farm for power outages.