Mm, thank you Kelly. I've done a bit of reading up on tank cycling as I never quite understood it before, and what you said makes perfect sense, and provides a clear explanation for the problem. Thank you for your input.
With the 10 gallon tank now, along with the gravel vacuum and much more broad tank opening, filter, etc., water changes and typical maintenance should be much easier, and I hope for a full recovery soon.
As well, I have planted live plant bulbs under the gravel to grow and provide natural filtration.