[Ed.]

Hi, As an example, I recently got rid of my breeding tanks, but I did build my own stands for all of them, The biggest 2 layer stand (5 ft long and 4 legs) I built was out of 100mm x 50mm x 2.5mm RHS Galv. and it carried 1 tank (48"L x 22"H x24W" ) top layer and 2 tanks (24"L x 22"H x 24"W) bottom layer. I think there was approx. 1000 lts of water in all plus the weight of the tanks, so all up about 1200kg (about 2600Lb). Tanks were out of 10mm glass. The stand had 2 levels with the shelves having 3 cross bearers (50mm x 100mm x 2.5mm) plus the 2 ends 100mm x 50mm x 2.5mm, a layer of 12mm ply and a layer of 10mm styro foam as compressive padding (very important) under the tanks.

I only used this steel as it was scrap from building my garage.

Now the problem in your case isn't in the vertical steel legs not being able to support that weight, it is in the horizontal steel flexing. Having the center of the stand reinforced with a middle support leg will virtually eliminated all flex of that distance at it will only be a 4ft distance between the legs.

Now just using the size RHS that I used, I could have made that stand another 2 layers higher and the legs would have still supported double that weight comfortably, steel RHS (and angle) under compressive force is much stronger than most people realise especially since you are using 1/8" thick. (3.175mm).

3-4 ton spread over 6 legs is not really a lot of weight for that steel size that you are planning to use. Also had a 6000lt (Approx 6.6 ton ) deisel tank sittting on 4 x 1 mt legs (75mm x 75mm angle x 6mm) once.

Another tank stand I built was for a 1.5mt L x .6mt W x .685mt H 630Lt aquarium and it was out of 50mm x 50mm x 2mm SHS steel and 1mt high. 6 legs and braced with some 25mm SHS shelves and 50mm x 35mm x 2mm on the ends. Total weight approx 2000lb
and that is out of 50mm x 50mm x 2mm steel, these are just to give you an idea. Even this stand is overkill.

However I would prefer to use 100mm x 50mm as the horizontal bearers as they will bend less than what you are planning to use but either way I cannot see a problem, just go into a few local aquarium stores and have a look at their stands and I think you will be suprised at what some use. Just remember to prime it and give a good coating of paint as rust will eventually show if you splash water over it. Weld the steel all around as you don't want water getting in to the joints. If you are putting on concrete floor, weld and seal the bottom of the legs with the same size plate ie; 3"x2" , if on a wooden floor, then sorry, I can't advise you as you will have to get someone to work out the loads, but it will need significant underfloor reinforcement as most houses or wooden floors aren't designed to take 3-4 ton. Sorry about the long rave and mixing imperial and metric measurements.

Regards