Terracotta, being very porous, would eventually dissolve - yup, affecting pH - or maybe just crack, split, etc. leaving other fishy dangers. I would try to steer clear of clay. Clay + Water = MUD! This is why, when you buy a terracotta dish or plates, they tell you not to put it in the dishwasher - even if it is glazed - there is usually a ring around the bottom of the plate that is not glazed where the plate was standing when it was in the kiln... an Achille's heel, so to speak.
I have seen lots of tubs that look like terracotta but are not - but never in small sizes. I'll look around. I'm working on adding plants, too!