I can only have 1 fish in the aquarium?

[Binski]

when i'm holding a fish while there is another fish in the tank, it shows ‘E to insert fish", but it will first retrieve the fish in the tank when I do so. And then I get to put the fish in holding im the tank

[fdjshrdisd]

As far as I know you can only have one fish in the tank at a time, if you are on a sever with others then everyone elses single fish will show up as well.