Great info - many thanks. Just to confirm, my tank is ali, is 2.5" diameter. Will see how it looks on it's 5 year birthday and then decide to replace, get it checked, or risk keeping on using it if it's still pristine until someone questions it or it gets to 10 years old, when I will get it checked - I'm guessing this could be normal practise for a lot of folk. I fill it myself with a stirrup pump as I like the exerice!