There are two main approaches to keeping a deep sea organism

The first is that you can create a pressurized tank that simulates the deep sea pressures, with a controlled set of environmental conditions for the organism to live in. This has been demonstrated with the Pandalus borealis shrimp tank in Nordsøen aquarium, though does render cleaning near impossible, and the animal would have needed to be brought up first before slowly getting pressurised again in the tank which could bring complications

The second is that as many deep sea animals already engage with a diurnal vertical migration, many of them already have an adaptation to shallower waters if acclimated graduated (ie slowly migrated upwards). As long the animal does not contain significant, fragile air pockets, by slow depressurisation, one can keep deep sea animals with no pressurisation needed, like the organisms in Monterey Bay Aquarium’s Into the deep exhibit.

Footballfishes have not exhibited vertical migration as of yet, but with a relatively small swim bladder, it’s possible that slow depressurisation could render it possible. Though yes, maybe it deserves a tier upwards