They are very sensitive little fish, easily intimidated and like cover by dense planting very much.. I have a few (started out with 15) of them in a little 60x30x30 tank for 18 months now.. I saw them develop from a sparsely scaped nature style tank into a a densly planted jungle. ANd still in the jungle they are sensitive and very easily intimidated and like to hide and dive into the vegitation.. They are also very sensitive to flow if it is to strong and not evenly dispersed by the setup of hardscape they might all group up in a corner where the flow suits them best..
Taking all these aspects into account
than imho this is not a suitable sp. for a wide open high flow, bright light, high tech iwagumi style tank. I assume with Iwagumi a few rocks with mainly small carpet plants and maybe some contrasting stems in the back.. And seeing how easily they are spooked an educated guess tells me they will not feel very safe even when cramped up with a large school of 50 without any sufficient cover. The more you have the less cover there will be for them.. You probaly end up with a rather large school constantly nervous little fish not knowing where to go.
Tho i noticed some improvement in their behaivor after introducing a small school of black neon tertras
these are a tad bigger and much more outgoing, bolder and more agressive.. Because they are so closely related they school together and the Embers also became more outgoing and mimiced the black tetras behaivor.. The black tetras agressiveness pulled them over the line to also be more agressive and compete and triggered competition. But still when i clean the tank all embers are into hiding in the plants and the black tetras hang around my fingers to pick up anything i swirl up from the substrate.
Over stocking a wide open iwagumi style setup, needs already a lot of flow to compete with the polution the overstocking creates.. This will not favor the embers behaivor and natural instincs. These are not realy suitable fish for such a relatively open, bright, high flow setup. I also got a 90x35x35 tank, a tad smaller then yours, but if i imagine 40 ember tetras in there, it would be pretty over stocked. Technicaly this could be filtered, if the fish like embers would like such an inveronment remains to be seen.. Personaly i doubt that a bit.
I would go for a sp. naturaly living in a higher flow clear stream.. If you would place iwagumi in the nature style, i clear higher flow stream would be the best comparisson.. Why put a fish in there that lives in moderate flow and rather near dense marginal vegitation. (?)..