In 3 turns, Puffer Fish is dealing 5 damages a turn, just like Toadfish (including poison damage). Additionally, this damage stacks; if Puffer Fish is removed from play, the poison damage continues stacking up.
And if Puffer Fish lasts more than 3 turns, its damage continues adding on past 5 damages a turn while still remaining cumulative. 5 
for such a powerful beast is fine; in addition, lowering it to 4 will make Fractal + Puffer Fish a very dangerous deck.
Puffer Fish. Cost is 5
Turn 1: Puffer Fish does 2 damage, poison does 0. Poison Counter++. Total Damage: 2
Turn 2: Puffer Fish does 2 damage, poison does 1. Poison Counter++. Total Damage: 5
Turn 3: Puffer Fish does 2 damage, poison does 2. Poison Counter++. Total Damage: 9
By turn three, the Puffer Fish has done 9 damage with arguably lasting 2 damage a turn; easily mitigated or ignored by many decks. In contrast, the Toadfish has done 15 damage (5 ATK * 3 turns, no poison) and presented a far more dangerous threat to other creatures.
Flying Arsenic (Elite). Cost is 2
, 1/0
:
Turn 1: Arsenic does 4 damage, poison does 0. Poison Counter++. Total Damage: 4
Turn 2: Arsenic does 4 damage, poison does 1. Poison Counter++. Total Damage: 9
Turn 3: Arsenic does 4 damage, poison does 2. Poison Counter++. Total Damage: 15
I think the stats still speak for themselves, especially considering Arsenic is just as effective with FW as it is without, though its creature spam ability is limited as there is no FW on a stick. As to more deck viability, yes, an Arsenic deck would require FW, but in a deck of say 6 Deadly Poison, 6 Arsenic, 6 FW (unupped or upped, it doesn't matter) and 12 Death Pillars/Towers (Mark Air/Death) you're almost guaranteed the combo you need, whereas a Puffer Fish does not require a second card, significantly higher resources and quanta (usually Aether) must be invested to see an increased return. I just don't see the cost effectiveness of a Puffer Fish, especially when it can be far more easily dealt with than an Arsenic, via EE or Diamond Shield or any other host of methods.
To be fair, my largest experience is with PvE, so this may change some with PvP.
And I wouldn't mind my Fractal Fish deck to be a bit more viable...
