Not really, unless you're willing to stretch sustainable to the breaking point.

As I understand it, after a few years the floor of the fish farm is basically a solid toxic layer of fish food and fish shit, and it becomes unsuitable for farming. That's not sustainable.

There is nothing wrong with the process, just the implementation. The bottom layer is not radioactive, it just needs to be processed properly. Since there is not going to be anything there of significance which is not organic there is not reason you could not move fish from pond A to pond B, and then process the bottom layer of the pond to be fertilizer that will help grow more food to feed to the fish, etc. Since you are talking about years of use in a single location it is pretty obvious that you are not actually talking about known sustainable processes but are tagging that label onto something else.

I think "properly processing" a 3 foot thick layer of sludge on the ocean floor might be a little more difficult than you're making it out to be. Or maybe it's easy. I don't know.

But as things stand right now, the process as implemented is completely unsustainable.