Not a clue what ADA uses. It all really comes down to density though. The stuff you see at big box pet stores is called mopani, it sinks every time, and I think it is from Africa (not too sure though).
If you were down south you could collect cypress. "Sinker cypress" is typically recovered from stream/rivers and makes great DW if you are lucky enough to be in an area where it occurs naturally.
In general, any old, long dead, hardwood will work (so long as you spend the time to water-log it). But finding pieces that are dense enough to sink on their own, in a dry state, is quite a challenge.
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