Natasha, if you are in the Perth metro area, try Veba Pet Supplies in Zeta Cres, O'Connor. They have black worms in regularly or did have a couple of months back. Saturday mornings are usually the best time to go. They also have meal worms. When I feed black worms, I hand feed which can be messy but works. Also I am interested in where you got your axolotl as I have sold a few in the Perth Metro area?
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Regarding size, my two axys I have had for about 8 years and have only noticed over the past couple of years that they are getting good and round as opposed to long. I don't think there is a standard suffice to say that when they are sick they usually look it.