Maybe change the effect so that it permanently decreases maximum health up to 20 points but cannot lower actual health?
air already has a card similar to this, unstable gas.
That's an interesting suggestion. I'd have to make it a bit cheaper then if it won't lower actual health, since it would make this card less versatile. :p I would be fine with doing this, BUT
I can't make the effect permanent. Making the effect permanent allows multiple Asphyxia's to stack, in a sense. If you read the first page of discussion, we were talking about how UG+Asphyxia could a problem. The problem below was solved when I declared that only one Asphyxia's effect would work per turn.
5 Asphixias kill you in 1 turn, and it's 25/20 . You could argue that you would have to draw 5 out of 6 of them to win, but then you have Unstable Gases, so you would only need 5 cards out of 12 to win.
Unstable Gas deals 20 damage AND 1 to every creature, but it's a Permanent that uses (duo) and you have 1 turn of cooldown.
Asphixia deals 20 damage and reduces max hp, but only lasts for 1 turn.
Each one is balanced on its own. But in my opinion having both in the same element might be overkill. As I said, I like the idea, and I don't want to change it, so I hope I'm wrong.
Of course you have to use them in the right order; but someone trying to use this strategy probably is going to realize that, and UG's are permanents; they can be put on the field if your hand lacks space and be ignited later.
If I can't make the effect permanent, making this card lower only the opponent's maximum health and not his/her actual health would make this card too situational. For one turn, your opponent's HP drops from 100 to 80. Unless you can deal 80 damage in that same turn, why would you use this card? By the time you get out 5 Dragons and a Sky Blitz, the opponent probably has less than 80 HP already, and you're better off just using your creatures to attack.
So if I can't make it permanent, I can't change the effect either.
Considering the first version, yes they're similar, but they're also so different in so many ways. You're not going to use Asphyxia when you know you won't kill the opponent in that turn. You're not going to use UG's when your crucial creatures are going to die.
This is probably a horrible example, but consider Horned Frogs and Cockatrices. One is 3|3, the other is 4|4. They both have no abilities, and they're both in Life. From what I can see, these two cards are even more similar to each other than UG and Asphyxia. What makes these different from UG and Asphyxia?
