It's been a little while since I updated this topic with any progress but I have some news that has accumilated over the last few weeks.
I was never able to get a second cube tank for Kaze, it didn't come into stock and looked like it'd take a long time.
Echo unfortunately passed away a couple of weeks ago; he fell ill very suddenly and simply didn't make it. I did everything I could to help him (he developed dropsy) but nothing worked... this left his tank empty and a space in my room that felt, well, empty too.
I decided to tear Echo's tank down and set up the 54 litre instead for Kaze. After spending lots of time and money on getting the tank just-so the bottom seal gave way and the whole thing started losing water. It wasn't something I was able to fix so I decided to clean out Echo's 7gal (this cube aquarium actually holds 27 litres/7 gallons, not 21 litres/5 gallons as I thought) and use that instead.
After a lot of heartache and frustration I feel confident that Kaze is finally in an ideal home, he's much more active than he used to be (although he's always been rather active) and just seems happier and healthier. It's taking a while for his new piece of driftwood to sink but in the meantime I've got a plate and glass measuring jug sat on top.
Hopefully things can calm down a little now and I can look forward to watching Kaze live the rest of his life in my favourite set up.