Fluid wagon capacity is affected by quality, but as of the current mod version 1.0.1 storage tank capacity stays the same. I double checked with and without other mods, as well as filled the storage tanks.

When you hover over a placed storage tank with higher than normal quality, what mod does it say it's from?

Only base mod

The mod settings are default, with "Enable Storage Tank changes" checked

I think I know what the issue is...

Hopefully fixed in 1.0.2

Sadly 1.0.2 does not fix it

Tried with different qualities, on different surfaces, different fluids, and on a new save

Right, I forgot about existing saves. 1.0.3 should fix that as well.

Sorry, still no change. Again, tried different qualities/surfaces/fluids, with new world and and existing saves.

In that case, please show your mod list; I just verified that it's working in a new world with no other mods than this enabled, so the issue must be some interaction.
I probably won't be able to find the solution today.

I have tried with just the base game, quality mod, and editor extensions. What else could be different from your setup?

I'll upload a new save when I'm back at m workstation of that will help you debugging.

We are talking about the storage tank, right? Not the fluid wagon, just the vanilla fluid container which usually has 25k capacity

Yes. I booted up Factorio, disabled all mods (except the 4 vanilla mods), downloaded the exact zip from the mod portal, launched a new game, "/editor", and placed down a couple storage tanks, which when connected to an offshore pump filled to 40k(rare)/62k (legendary)/25k (normal).
So maybe editor extensions is actually the problematically interacting mod??

Indeed editor extensions it was.

The entity/item tooltip only shows base game (and SA) dependency, no other mods whatsoever.

I double checked existing saves, and already existing storage tanks are unaffected, but newly placed ones work just fine.

The tooltip info does show 25k capacity for all storage tank qualities, but the actual capacity is higher. Strange enough, the tooltip info for the fluid wagon does show the increased capacity.

And also isn't it weird that everything else that your mod touches works with editor extensions? Including the fluid wagon and roboport charge points.

Btw, it seems the charge points overlap with the normal four ones, or maybe just the charging speed is increased, it's hard to tell apart.

https://mods.factorio.com/mod/EditorExtensions

Okay I reactivated Editor Extensions and it still works. I get the feeling there was some kind of caching issue going on.

Replacing existing entities isn't implemented yet, and would cause installing and then uninstalling the mod to remove all quality entities where replacements were necessary.

Capacity for wagons and roboport charge pads are implemented with engine switches (the quality scaling is implemented in the game, I just enable it); for storage tanks or locomotives, this mod creates extra entities for each quality level and silently switches them out. This is why the former cases show the quality effects in the tooltip, while the latter only show the correct values after placement.

I just tested my mod with EditorExtensions, and storage tanks still scale correctly. How were you placing down the tanks? Just selecting the item from your inventory, or using some other method?

edit: even in a new save started with EditorExtensions, storage tank capacity scales for me...

Yes, now it scales for me too. I created the storage tanks via cheat mode, and via assembler. I can't reproduce the issue unfortunately...