I have aquatic soil in my horsetail pot in the fish tub. It's a clay product. Quite often out of stock but it seems it would be good.
Appx. same size as lava and doesn't dissolve.
Reddish color.
???
Do you mean fish tank gravel?
Regular fish tank gravel is just painted or sometimes not painted rocks.
That would not be appropriate for bonsai as it is just gravel. Gravel does not retain moisture or nutrients and its very heavy which would make moving trees around even more of a chore.
I do know that there is a lava product that is also used for fish tank gravel. This product would be fine as part of a mixture as long as its very close to the same size particles as the rest of the mix.
The mix most professionals use for most trees is a mixture of lava, pumice and akadama. The reason they use a mix is because not one of them alone has the moisture, nutrient retaining attributes that make for the best soil mix for most trees (notice I say "most trees" there are some species, that do better in other soil types or mixtures. Akadama is better at retaining moisture and nutrients than lava so I would disagree that is is a substitute for lava.
There is a referece article about the specific attributes of each of the different soil components done by markyscott
It is well worth reading even if you dont understand all the technical details, youll learn a lot
Here is the second resource developed from the thread "Introductory Soil Physics". It is a document explaining how the important physical properties in soil and substrates combine to control the amount of air and water in the pore space of soils...
www.bonsainut.com