I'm sorry that you've lost your frog and cannot speculate about the cause, which is impossible for anyone here to determine. I will, however, offer the following comments on what you've written to try and stem the flow of misinformation that Internet forums like this sometimes create.

Quote Originally Posted by FrogMinch View Post
He said that Bloodworms ( not exclusively my little frog's diet, but probably about 90% ) were so high in iron that a bloodworm heavy diet would eventually lead to iron poisoning and he thinks this is what killed the little guy. I've not heard that before
As far as I know, there's no substantial evidence to support that notion. Although there's some controversy amongst fishkeepers about the wisdom of feeding bloodworms, they're only likely to pose a serious threat to small fish that cannot digest or excrete the chitin, which the bloodworms' exoskeleton is made up of. I kept Hymenochirus frogs for around 20 years and never experienced any problems feeding them bloodworms and neither did anyone else I knew personally who kept them.

Quote Originally Posted by FrogMinch View Post
I appreciate that having a variety of food is important, and we had actually started to put amphibian vitamins into water once a week
I have no idea what amphibian vitamins consist of, how the frog could utilise them effectively or what possible adverse effect they may have had on the water chemistry. If you don't know either, they may not have had the beneficial effect intended.

Quote Originally Posted by FrogMinch View Post
I used to mix up the food a little between the usual frozen stuff ( he also said don-t feed him frozen, they lose nutrition that way )
I agree that live food is preferable to frozen food but, again, I kept these frogs for a number of years when their staple diet was frozen foods. I used to alternate between frozen bloodworms and frozen Artemia. Not only did they thrive on that diet but they also produced viable eggs. So, again, based on many years of personal experience, the advice that you've been given is questionable because there's no evidence of any significant nutritional deficiency as a result of freezing that I'm aware of.

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