Wow, you make it sound like this is the worst thing imaginable....I might have to come up with something DIY...
I hate to break it to you, but you are using a kiddie wading pool...to keep tropical fish...and have a vegetable stand modded and standing in there to grow a potted houseplant. You are already about waist deep in the horror that is DIY; any further and you will need a snorkel. Welcome to the dark side.
I like your latest evolutionary step in this project. Just two suggestions: first, if the bottom of the veggie-stand legs look like the top, then perhaps something to protect the pool liner from those cut edges might be in order? Maybe stand them on a piece of tile or plastic or something similar, something with smooth edges. You might also consider capping the bottom of the legs with some sort of foam foot for padding.
And second: those legs sticking up in the air like that are just begging to have you cobble together some sort of framework to act as a support for the pothos vine as it grows. You could make it so that it extends outward on all sides for several inches, perhaps even reaching to the side of the pond and fastening there as well. This would stabilize the whole thing exceptionally well against pacu persecution. You could also have this framework extend upwards to increase the space available for the plant to spread.