Fish Tank Issue?

So I have recently just acquired my first house, and I put a tier 3 fish tank into it. I caught 4 small fish- 2 copperfish and 2 doman bubble eyes- which are each supposed to take up 1 fish capacity. My tier 3 fish tank is said to have 4 fish capacity. Yet after I put in 2 of the doman bubble eyes and 1 copperfish, I could not put the last copperfish in and the last slot to put fish into had a grayed out X over it. I wasn't sure what to do so I relogged and it was still there. Has anyone else come across this problem? And is it even a problem or am I just doing or reading something wrong?

You can have as many fish in an aquarium as it's tier, so a tier 3 aquarium can hold 3 fish. The capacity is just used to limit sizing of the fish in addition to the number of fish.