They may well do. I have heard of people returning them successfully and quoting the 'not fit for purpose' bit of the Sales of Goods act. I'm a bit clueless about the law but as far as I understand, anything that can hold water and a fish even temporarily can be sold as a fish tank, even if it's just a large vase or bowl. Obviously this kind of thing would be shit long-term but I don't know how this will affect it being 'not fit for purpose' since there are no hard and fast rules about size to protect the fish or consumer.
With the RSPCA guidelines and also the 5 freedoms printed out, and the Sales of Goods act and 2006 Animal Welfare Act then you should be OK, but they may give you store credit instead of a cash refund.
Have a look here and also here and there's this from the RSPCA which is specifically about this type of fish tank.
The following is from the 2006 Animal Welfare Act, you can find it here
Duty of person responsible for animal to ensure welfare
(1)A person commits an offence if he does not take such steps as are reasonable in all the circumstances to ensure that the needs of an animal for which he is responsible are met to the extent required by good practice.
(2)For the purposes of this Act, an animal's needs shall be taken to include?
(a)its need for a suitable environment, 15 litres is too small volume wise and swimming space wise
(b)its need for a suitable diet, not affected by biorb
(c)its need to be able to exhibit normal behaviour patterns, behavioural problems can happen in overcrowded conditions
(d)any need it has to be housed with, or apart from, other animals, shoaling fish should be kept in groups of 6+ really, no space for this in a 15l biorb so if anyone suggests 'a couple of minnows' then that's no good
^and
(e)its need to be protected from pain, suffering, injury and disease. ^ fish more susceptible to disease in overcrowded conditions
Here is a list of the Five Freedoms from the RSPCA I would say that all of them apart from the first will be affected by sticking fish in a 15 litre Biorb.
Good on you for doing your research and swapping it for something better. I do wish the manufacturers were a bit more responsible about what they sell but I think they just see a bunch of little swimming dollar signs instead of live animals.
Good luck, let us know how you get on :)
Argh, I can't get those bloody italics to work. Never mind.