I have some of the rubbermaid stock tubs, much like 2nd picture in post 3 above.
The 50ish gallon one is very shallow. I have two that are 100 gallons, quite deep.
They are great for growing out fry. When used outside, there's almost no visibility (algae usually takes over in the sun)
Inside, not too hard to keep the water clear, but still not nearly as enjoyable as a tank.
If you want to keep your frontosa maybe just as breeders as opposed to enjoying watching them, a big tub would probably be ok (although it would be hard to monitor the breeding pairs).
I like the tubs to grow fry outside in the warm months. They are also great for emergencies (like leaking tank or temporary housing/quarantine)
I wouldn't want them as a permanent inside housing though. It can be hard to see a dead fish on the bottom of one of the deep tubs.
Also, these things have a pretty big footprint. take up a lot of floor space.
I also share the concern about the performed pond liner needing dirt around it to hold its shape. Use a livestock water tub if you are going to do this..
