Why the limit on Aquariums?

[Vrankyl]

It has to do with the loading of these items. Stationary items that are not customizable contain far less data to load. Then interactable items contain even more information. Finally, special items such as flower pots, npcs, mannequins, and aquariums take far more information. For the longest time while there was no official cap on interactable items there was a set limit to how many interactable items such as food, certain lights, showers, and other such items could load at a time. If you had more than that in a given area only some of them would load and a lot of items would vanish from sight until you were right on top of them.

It took years for them to be able to work around that, and this was even after the initial item limit increases for housing. Aquariums and other items like that require FAR more information to load, and so by allowing even more of them you'd end up having rendering issues just like used to be caused by interactable items. And I'm not even talking a few items disappearing, I am talking like half of a floor vanishing from sight in some cases with the old thing. With these items it could be even worse. Now it could be worth asking them to do as kaynide suggested and making it a cap for all special items like that and allowing people to chose. The only issue I could foresee is that each of these different items have their own unique data load, which is likely why they each have their own limits balanced within the limits of others to prevent issues.

And just a quick edit. The reason why Apartments and FC rooms have a lower limit is because they are all one floor. The game only has to render a single floor at a time for each player, so the moment we're out of sight of the first floor it no longer needs to render and can be made dormant.

[EZGIL]

pretty sure Large is 10.
the medium I control has 8.

also, my tanks seem to grafix glitch from time to time. The area around certain tanks goes black. (but doesnt crash anything)

[Rongway]

The limit on aquariums has to do with them being transparent and how the game isn't that good at handling transparent objects.
From Thread: Letter from the Producer LIVE Part XXXI Digest (2016/09/13):

Aquarium
As it is difficult to implement transparent objects in an MMO, it took us a while to add this feature but we are pleased to announce the addition of aquariums. They will come in several different sizes, with the bigger aquariums being able to accommodate a larger number of fish. Maintenance will be necessary, but hopefully well worth the effort, as different varieties of fishes players can put in them will gradually increase with future patches.

The PLL Digest doesn't explicitly say this is the reason they are limited, but that's why they're limited. In particular the game doesn't like it when transparent objects intersect (Yoshi-P mentioned this in a PLL or interview--probably untranslated commentary in that same PLL, but I can't remember for sure) and they want to limit the number of possible such intersections.

(This transparency issue is, by the way, also the reason all the combat effects besides ground AoEs are opaque and nobody can see anything through caster LB3)

[LianaThorne]

I'm in a FC. We're still stuck in a small. This fixes nothing. And I'm not looking to decorate a FC house, I want my own. So, again, this solves nothing.

Move to Materia then. Apparently there are larges just sitting there.

Problem solved.

[LaylaTsarra]

Used to be only 1 per then they increased it. More would be nice but I think it's really more of a loading issue for people who enter your house. I'd personally like to see direct lighting fixtures increased from a limit of 2 which I think isn't anywhere near enough.

[Naoki34]

Why is there even a restriction on this? My apartment has space for 6 Tier 4 Aquariums along the edges. So why are you limiting me to 4? If I want more, I have to get a Large house, which is literally impossible.

You're stymying my creativity, and it's lame, and I hate it. Remove the limit.

I have room, let me use it: https://imgur.com/JGbDMQQ

There's no point in having a big house except to say that you have a big house.

The ratio of furniture slot to size is a medium.