Is There Any Substitutes For Underwater Bonsai Trees?

Why Can’t Bonsai Trees Live Underwater?
Bonsai trees are genetically identical to a normal tree, only small due to having the roots constricted in a tiny pot and being continuously pruned. Normal trees have stomata in their leaves, which is, in easy terms, how they breathe. Being that bonsai and a normal tree are genetically indifferent, this too is how a bonsai would breathe. This leaves both trees unable to breathe at all underwater, meaning they will die very fast.
When roots are fully submerged for a long time in water, they are very susceptible to ‘root rot’, a disease that effects plants or trees in extremely wet areas. When a bonsai has root rot, you can try aggressive pruning of the roots but depending on the condition and health, it could be fatal to your tree. Yet another reason why it’s sadly impossible to grow a bonsai tree underwater.
Pruning A Bonsai Underwater
When people think about having a bonsai tree underwater, they normally gloss over the fact that bonsai trees only look so pristine and well formed due to the constant pruning that goes into them. Unless being left to grow out for something like a sacrificial branch, bonsai trees are usually lightly pruned every week or so in the growing seasons if on display. Whilst this isn’t actually a problem, it would definitely something to think about if you don’t have too much time on your hands – this is why I would lean more towards the implementation of a ‘fake’ underwater bonsai instead.
Final Thoughts
As of right now, I’m yet to find anyone who has managed to successfully grow a bonsai tree underwater. Growing a bonsai semi submerged maybe possible depending on the species but for the vast amount of tree species that wouldn’t even be do-able. Tree foliage and root systems will quickly die and start rotting away if grown underwater for a short amount of time. When a bonsai tree is over-watered, the roots have no access to oxygen it needs, causing them to die and break down. This process is only going to be accelerated by growing the bonsai fully underwater.
A great alternative to growing a bonsai tree in your fish tank will be to use some driftwood or even the trunk of a dead bonsai, combined with some light moss. If done well, this will give the illusion of an underwater bonsai tree, which as of right now, is the closest you can get!