a quick search brought up that all species of Elm
(American elm (Ulmus americana), English elm (Ulmus procera), Rock elm (Ulmus thomasii), Slippery elm (Ulmus rubra) & Wych elm (Ulmus glabra))
are classified as a hardwood.
therefore, provided it is not green, it should be ok.
my only concern would be if the Elm had Dutch elm disease or more specifically the fungal spores that cause it,
would they somehow be dangerous in an aquatic environment? I don't know.
More specifically the wood in which I was intending to use is chinese elm, a dead chinese elm bonsai tree which died when my neighbours forgot to water it when I was on holiday. Rather than put it to waste I thought it could be a little feature in my tank. As far as I know the tree had no disease, but I'm not sure how you can tell, it died from lack of water as is the case with bonsai trees when they are only kept in small pots with a limited water supply. I doubt they would be dangerous in an aquatic environment as they would probably be dead, no? If I boiled the tree before do you think it would be OK?