Thanks madeve. We didn't realise it was this complicated!! The pet shop where we got our axies (which we will no longer be using!) has given us all sorts of bad advice. Under their guidance, we have been removing any large waste daily, changing approx 10% of the water every week and completely striping and cleaning the whole tank around every 6 weeks (thoroughly cleaning plants and substrate as well as wiping down the whole tank and refilling) I'm guessing this is at least part of the reason our poor axies are unwell.