Great pick for Druid's first aquatic Wildshape! 

its a piranha

Your not wrong. 

Correction, it is a cold-water piranha. But common D&D really? You couldn't just call it that?

If you turned an opponent into a quipper on dry land using polymorph, how long would it flop around before it died from suffocation and what would happen when it did. Does a character drown as a result of hit points being reduced or does it just cease living because it can't breath?

My friend died from getting attacked by a quipper

Quote from DavidP2K >>

If you turned an opponent into a quipper on dry land using polymorph, how long would it flop around before it died from suffocation and what would happen when it did. Does a character drown as a result of hit points being reduced or does it just cease living because it can't breath?

 

A creature that can't breathe starts suffocating. In this specific case, the quipper can hold its breath for 30 seconds (that is, if the DM deems that quippers can hold their breath), and starts choking after that. It chokes for one entire round and starts its next turn with 0 hit points.

Additionally, polymorph says that "transformation lasts for the duration, or until the target drops to 0 hit points or dies", after the creature has spent the one round choking, it would revert back to its original form. Whether that means the opponent is dead or not, is up to interpretation.

It's really not up to interpretation.

If you die in a Polymorphed form, you revert to your regular form, with the HP you had before. You only take the carryover damage.

Polymorph doesn't let you turn someone into a newt and kill them easily. It lets you turn someone into a newt and easily capture them, sure.

In this case, you can catch the choking quipper that used to be your enemy, and then put it someplace safe before it dies and turns into its real shape. You'll want to be careful using that.

True. Apparently I didn't read the very next paragraph from Polymorph's description.

You could place said morphed Quipper into a small container/cage then kill it, the change back into it's original larger being while inside the small container might just kill it. The container/cage would have to be strong enough not to break or have sharp edges so that the enemy would die.

Why not just call them Piranha?...

killer fish from san diego

Killer fish from san Diego

I want one

Quippers seem to be more like the bream analog to piranhas. Likely the reason they can live in so many different environments isn't one naturally hardy species but various species that can survive in different environments.

Quippers are probably used instead of piranhas to allow for the stereotypical bloodthirsty piranha to work as a D&D creature; Actual piranhas are very timid and don't really attack people

killer fish from san deigo

Can I use this as a template for a leech

Killer fish from San Diego 

110 of them and the players beg for mercy.