Adding Another Tank to Shelving Unit?

[Silhouette]

I currently have a Fluval Flex 9gal sitting on the top shelf an Ikea's Fjallbo Shelf Unit, which you can view here for reference:
https://www.ikea.com/ca/en/p/fjaellbo-shelf-unit-black-50339292/

I've had it up and running since roughly early October 2020, so pretty much 2 years now, and it's been sturdy the whole time. The site lists the weight limit as "Max load/shelf: 75 lb", so I'm guessing that's 75lb per shelf. I'd love to get my 5gal up and running, and was contemplating if it would be okay on the middle shelf, underneath and opposite end of the shelf from where the other tank is. For those of you who would have a better idea of these things, does that sound safe to you, or do you think that would just be tempting fate?

Thanks!

[meadeam]

without testing the shelf myself I can't be sure, but from your description I would say it should be fine.  In this case you shouldn't need to worry about the shelf being able to support the weight vertically.  My concern would be if the shelf will twist, or deform in any way.  That is what will make it fail under load.  If it doesn't flex in any direction at the vertices, then it is solid.  Again though, it is hard to describe how much twist is too much without seeing the shelf.  I shoot for near absolute rigidity.  You especially don't want movement if the weight is held high on the top shelf.

 

[TOtrees]

I also think you'll probably be okay, but I tend to think of this from the perspective of "what are the likely modes of failure?" eg HOW is this likely to fail or collapse or fall, if that were to occur. This allows you to look at specific aspects of its construction/location/setting, and see if you need to change or improve anything. 

75lbs per shelf is low, compared to the weight of your 9g and a future 5g tank. Generally, 10 lbs per gallon will give a low ballpark for what you're dealing with, so say 150 lbs for your two tanks, at the low end. Call it 200lbs to be safe?

What is it about the shelf unit that contributes to that low weight rating? Is it the sag/bend in the middle of the shelves? You could mitigate that by placing the tanks more over the uprights than in the middle of the shelf. Is it the screws/welds that connect the shelf supports to the uprights? Are they likely to shear under this load? There's no fix for that (unless you want to add strength or reinforcement eg welds or bolts). Or is the leg construction thin metal that could twist/crumple under too much weight?

Some floors might bounce or sag when you move through the room, so adding a bit of movement to the shelf, which can be increased by water sloshing a bit, might make a tolerable problem more serious. If the floor isn't level, the shelf might lean away from the wall a bit, which could also be a problem. This could be fixed by attaching the shelf to studs in the wall with screws and brackets. 

With all this said, I'd find say 150-200 lbs of weight (rocks, bricks, person, whatever), put the shelf where you're going to have it, load up the weights and give it a wiggle. Satisfactory? Needs improvement? Or not going to work no matter what you do?