I was always looking for 1x1 fluid buffer tank. Can you maybe make a non-finite version too?

yes. Go to your mod/save file, open prototypes/entity.lua with notepad, go to base_area = 9990000000000000000000 and like the text says there, 100=1 fluid. If you want 1mil fluid then you change it to 100mil. I cannot guarantee that you won't lose fluid stored in the tank, it might cut-off the high part (most likely) or it will simply scale it and keep whatever percentage of fluid it contained (unlikely)

p.s. if you change the mod, I suggest you rename it, to avoid other players not being able to join your game due to conflicting mod info

in 0.16 or 0.17 i had used some other mod for a small 1x1 tank. there should be two such tanks available for 0.18 now, small inline storage tank, and pipe meter (redux). i have just installed them together with this mod to be able measuring the level of fluids in my pipe network and send only surplus to the infinite buffer fluid tank, or get some fluids when the pipes start running dry because i tried to store too much in this mod's tank (but i have not used any of them yet).