It’s enabled in the mod settings. I haven’t tested with a higher-quality tank yet, but I do see the little blue icon that shows the different quality values for other things, like wagons. Perhaps it’s a mod conflict? I don’t see More Quality Scaling in the hierarchy list of mods for the storage tank either. Could it be a conflict with Call a Plumber or another mod?

https://i.imgur.com/hRl2PlK.png

Once you place down a quality storage tank, the capacity will be adjusted. The quality tooltips are hardcoded and can't be modified by mods - I could in principle add all the quality values to the item(/normal entity) description, but having 5 (or, with quality-adding mods, substantially more) additional lines in every item tooltip would get annoying very quickly...

edit: The reason eg. fluid wagons show the quality tooltip is that there is a corresponding feature in the engine (quality_affects_capacity in the FluidWagon prototype), while for eg. storage tanks, no such feature exists.

Actually, it does seem possible to at least add a tooltip field to entities that behaves like "real" quality tooltips - though there would still be the original tooltip as well.
Eg. for an uncommon storage tank, it would say: "Storage volume: ◆32k, Storage volume: 25k" (with the former showing all the quality levels when hovered over).

I will look into implementing these tooltips for all the affected properties.