Perhaps tie the availability of additional tanks to having completed the initial seven magic fish? So that people wanting to play on for the continued fun would not be challeneged but more in it for the breeding and beauty.

(Possible FT2 item?)

I did the same thing - a pair of each "cash cow". My cash cows were all magic fish as well - generally the biggest money makers - and it allowed me to keep all my magic fish!

Ahab, Look Here to see the fish Fish Tycoon community wishlist.