What is the largest tank you would put in an upstairs room?

[skjl47]

A 55, 75 and 125 would all have about the same psi being applied to the floor. This is because they are all the same height, the only thing being increased is the area. In practice the 55 and 125 would likely be the most stable because the increase in width with the 75 wouldnt necessarily increase the amount of support beams the weight is distributed across. For the record i have never once heard of anyone having a problem keeping a 75 gallon on a higher floor. Also im not an engineer, i just did some math.

Hello; You are talking point loads compared to spread out loads. Sort of like the difference between high heels and work boots. You walk across soft ground in heels and you sink in. While in some ways spreading a load onto a bigger footprint can help, I do not think it applies so much to the overall load a floor can support. First it is smart to run a load perpendicular to the floor joists and it is also smart to place the loads close to a load bearing wall.
I happen to like the iron tank stands better than the more common wooden stands. I do get the four legs of an iron stand make four point loads compared to the more spread out footprint of some other stands. However the total load on the floor structure remains the same no matter which sort of stand is used.