I just had a play with the CC and created a new fictional tank using Tank 2
Opened the CC page by selecting cherry barb from the fish profiles
Logged in to the CC - image CC 1
Added tank length and volume, and clicked save - image CC 2
In the cherry barb profile, clicked on "add the cherry barb to your tank", and amended the number form 1 to 6.
Clicked save/update - image CC 3
The CC added my tank details, added cherry barbs and saved the tank.
I then logged out and back in and the tank saved was still there. Tank 1 was the tank displayed, I had to select tank 2 to see the one I just created.
Did you do the same as me when you got the error message?