I think what they do is that since the tanks need to be maintained by them if they find dead fish in there they just take em out and add in new ones also how they get the tank cycled fish-cycle. I came up with this idea when they made a tank with sharks in it and they said they would re-home them when they got to big for the tank which was going to be quick since there were like 6 sharks in a 2000g tank. Or I at least hope that they do something like that, I believe I read somewhere that the starting price for these custom tanks is in the 20 grand.
I remember a few of the tanks on ebay the record player 1 and the phone booth.
1 M. Turgidus - 29 gallon