Are you planning on putting the new tank where the old one is, or someplace new? I'm going to assume you are planning on putting the tank where the old one is. This means you will need to temporarily store large quantities of water. If you are placing the tank elsewhere, then you can move a good deal of water from tank to tank. Initial steps: Get 3 to 6 - 5 gallon buckets - depending on how much water you can lift in a bucket. To give you an idea of weight, a gallon of water weighs roughly 8.34 pounds (Standard US) 5 gallons weighs in at 41.7 pounds. Get new gravel - rinse that out repeatedly before adding to the new tank. Rinse all "toys" (coral, diving men, treasure Chests, fake plants, etc). Along with the new tank make certain you have a correct sized pump for moving 30 gallons of water in a reasonable time (talk to a person at the store you bought the tank), if your old pump will work, might as well buy all new hosing - it is relatively cheap. 24 hours before you are ready to set up the new tank fill up enough buckets to give you about 20 gallons of water. Allow the buckets to sit open, stirring occasionally to cause the chemicals in the water to out gas. If you have needed chemical additives for your water before, now is the time to use those. Read the directions for them and if you need more time, then fill the buckets earlier. Place the buckets close to the area you will have your tank - this will allow the water to reach a close temperature too the existing. If you are using a heater that will require a bit more patience and time in the filling stage. 24 hours or so later: Fill 2 buckets with the tank water (from the old tank) move the fish carefully from tank to the bucket(s) depending on the size of the fish - you want them to have enough room to swim around, enough water so they can breath for an extended period of time without a pump. (10 gallons of tank water in reserve) If all can fit comfortably into one 5 gal bucket for a couple hours, then move them to one, putting a second bucket of tank water in reserve. That should remove about half of the water from your existing 20 gallon tank. Once the fish are gone you can remove another 5 gallons of water carefully. (15 gallons of tank water in reserve) ---> DO NOT stir up the bottom of the tank when removing water, your gravel or sand is there to collect biological wastes which will settle to the bottom of the tank through the gravel/sand medium.<--- That should leave you with about 5 gallons of water left in the tank - pretty shallow and close to the gravel. You do not need that water - remove or pump it out to a drain or out a window away from the footer of your house. Once you have drained out as much water as you can, remove the gravel, trees, rocks, treasure chest, bubble makers, filter, pump and hoses, blah. Remove the old tank. Set up the new tank. gravel, filter, pump, etc. Carefully pour in the 5 gallons of reserved tank water, add to that about 2.5 gallons of the "fresh" water. By dipping take about have of the water from each of the buckets that the fish are in - assuming you used 2 that would be about another 5 gallons. Your new tank should be about halfway full. Gently move the fish back in, and slowly and carefully add back the water from he temporary holding buckets. By his time you should have filled your new 30 gallon tank with all of the old tank water. About 15 gallons plus the 2.5 gallons of "new" water. Over the course of 2-3 hours gradually add the "fresh" water to the tank - about a half gallon at a time allowing the fish to acclimate. once your tank is full enough to allow th pump and filter to run turn those on. You most likely will have some water left over. If you are using a heater the process of introducing new water may take longer - you will add 1/2 to 3/4 gallon of new water then wait until the temperature gets to near the right temp, then slowly add more water.