Its the sun that dose them in. I binned a plastic 20lt just before Christmas cause it had faded to a pink and in one place white colour because it spends a lot of its life in the sun it was 6 years old and then I have others that are 20+years old that never see the sun and look like new.
I think with the age of the tank in question you have to remember that UV stabilised plastics have come a long way in the last 10years, before that they were a bit iffy
