The problem for me is how different these cards are. I think that really they should be two different cards.
For example, I think Toadfish should upgrade into Elite Toadfish or maybe Micro Toadfish ( 2
for a 2/3 creature with same ability-would need to be edited)
and then Puffer fish would be elite Puffer fish, and its unupgraded card would be ( 4:water, for 1/4)
This would make poison more acsessible for new players, and would increase the amount of cards in the game (GOOD!)