Why not move fish to a temporary home?
You’ve several larger fish that appear to displace soil/plants (earlier video) just by swimming close to the substrate, nevermind when they muddle about looking for food etc
You’ve put so much effort into this scape!
That MC should be rooted and growing in a couple weeks, and very much so by 6 weeks - that it’s still floating up indicates something is preventing this
Break up the MC clumps into much smaller portions, plant deeply (this is easiest in dry/damp soil if you’re willing to go that far)
Examine flow in the tank - it just needs to get everywhere, it doesn’t need to do so at a rapid rate - strong current can “lift” the soil and may be contributing to the MC’s lack of root development