i would use 4x6's and make sure that the top frame sits in the legs so that all of the weight goes down the legs instead of creating sheer force if you were to put the legs on the inside. Use backets and liquid nails construction adhesive too as well as lag bolts
take any of the larger stant build diy's bulk up the size of the wood a little more add some extra braces and have at it
alot of the diy's on here were people who had a need and found a solution it dosent hurt to ask ?s but at some point your just gonna have to go buy some lumber and go nuts
I have built several stands for very large tanks. My first question is what are the measurements of the tank? it makes a difference. People that say use 4x4 or 4x6 2x6 don't understand how strong wood is or isn't also welding up a stand isn't a bad choice...if you can weld. I have built and worked on wooden airplanes so I actually understand how strong wood is...my life and others have depended on it. Recently I built a stand for a 400 gallon that 2 people could carry (it weighed 150 lbs) and it easily carries 4000 lb of tank and water. Don't use liquid nails it sucks as an adhesive. But lets get back to the measurements of your tank, also how tall do you want the stand and what kind of filter you want to put underneath it. Also an important factor is what are willing to spend on the wood. If you want to go cheep then maybe standard 2x6 4x4 4x6 might be the correct choice, if you have some money to spend then some type of hardwood or poplar might be a better choice. Even though I have built some very large stands the last 2 were for a 240 and a 400. The 240 stand is built from 2x4 and 2x3 and cost under $100 to build and that includes the stain and sealer. The 400 stand is built from different hard woods mostly poplar and some maple but it cost about $500 to build.
I agree with people not giving wood proper credit. With stand like these, verticle support becomes the crucial part. Just make sure that you have the horizontal boards setting ontop on the verticles and you will be fine. If you are worried, go a little thicker on the wood. Many people have overbuilt stands. Once its covered, you never see it anyway.