But if you included this card basing the balancing on and not having any buff cards then it neither element could ever have any buff cards. It would be very limiting for future card design. Also I'd be wary of rainbows buffing this card.
In terms of abilities flying fish does have an ability, dive.
Your have a good point on the quanta cost analysis though.
Even if they did get a buff card, it wouldn't be a big deal. Think Pegasus.
Pegasus is 4/4 for 4
in an element that can give it +3/+3 for 2
. Now in my opinion, blessed pegasi aren't OP.
So what does this prove? It proves that a buff in-element on a diving creature isn't OP. Buffing a creature with dive essentially gives +2x/+y, where x and y are the original values of the buff. If that isn't OP on pegasus it won't be OP on flying fish.
Granted, the fact that flying fish has a quarter of the cost may have an effect, but we shouldn't say a card is OP based on cards that aren't indevelopment/developed yet. If a water or air based buff card comes, then we'll cross that bridge when we get to it.
About it having an ability, the thing is that the main point of damselfly and brimstone eater is to create quanta, with the damage just as an added bonus. The main point of the flying fish is to deal damage and it has no secondary purpose, so naturally, it should be much better at that main purpose than the damselfly and brimstone eater are at their secondary purpose.