Hi - from another angle here...

I got the impression that when you rented one of the informal terms was that you look after her son's koi.

Now it's, 'they're your fish, you pay.'

I think if they are YOUR fish now, you should rehome or sell a lot of them. Take them back to the pet store? Maybe keep one or two.

Go ahead and start that and THEN see how quickly they aren't YOUR fish any more!