Why are you going there? It has nothing to do with "Ethics"!
You people would love to put more cost and larger quarters for the frog when it just isn't necessary. Frogs don't have the same requirements as Humans or Fish. They do not need as much room period.
The Frog will be just as happy in 6.6 gallons as 10! I've seen Pac-Man frogs live in their entire life and grow to 8" across in a standard 5 gallon bucket.
Xenopus Laevis (Clawed Frogs) are not the same as normal frogs. There is very little about that that is relevant to the way we keep other more terrestrial amphibians. They are both predator and scavengers and very active swimmers. Yes a pac man frog does not need much space and they lie dormant for long periods of a time but comparing a pac man frog to an african clawed frog is retarded, they are nothing alike and their husbandry is COMPLETELY different.
These are fully aquatic frogs and their husbandry is much more akin to fish than it is to a pac man frog. Xenopus are very active swimmers and produce a LOT of waste. Why would you keep a frog in 5 gallons of water when it produces more waste than a goldfish? I think we would both agree that a goldfish would suffer and die in a 5 gallon tank... right?
You say your frog lived "quite a long time", did it live for over 10 years? Because these frogs have been known to live over 20 years if kept correctly.
I see a lot of posts on ACF forums about health issues like dropsy, bloat, red leg, fungus, burns, and I assure you that 99% of the time these problems stem from improper conditions people keep their frog in. Poor conditions stem from keeping these frogs in unfiltered and small aquariums. These are very hardy and long lived animals but every animal has a breaking point and keeping ACF in such small aquariums are a recipe for disaster. If you care about your animals, you should learn how to properly care for them.
To the OP. Get a 20 gallon long tank, a HOB filter rated for 40 gallons or more (whispers work OK), skip the heater (these frogs thrive at 68-72F which is room temperature), and keep a lid with no gaps on it so the frog does not escape.
If you want plants stick with floating plants like water sprite, amazon frogbit, wistera is also another good one. These frogs really like floating plants, you can get a little driftwood cave since these frogs like having a place to retreat to since they're skittish. Anubias and Java Fern works fine too, these can be attached to driftwood.
Overall these are not very demanding animals and with these few things your frog will be very happy and live a long time.
What I've suggested should be about the same price as your 'nano' tank and your frog will be much happier and you can enjoy your pet for years and years rather than watching it suffer and die after a year tops....
I have a very nice low-tech 'planted' ACF 40 gallon breeder tank and I will gladly show you some pictures if you want some good suggestions.