The "Euryhaline Curse"...aka "The Great Molly Misconception!"

eu·ry·ha·line (yo͝or′ə-hā′līn′, -hăl′īn′) adj. - Capable of tolerating a wide range of salt water concentrations. Used of an aquatic organism.

Like many of you, I have adored Mollies since I was a kid. 

They were probably the first "challenging" fish I had, as they were not quite as hardy as some of the Platies, Swordtails, and Guppies and such that were "standard issue" for hobby newbies. Now, I'll be first to admit that I am absolutely not anything close to knowledgeable on the family Poecilidae, but I have a fascination for some of the legendary fishes of our hobby, and for unravelling the reasons behind why we keep them the way we do. Mollies are one of those fishes that it seems like we've all kept, yet they have some sort of "thing" about them, as if there is some secret that we don't know, but should uncover. And there are a bunch of species that we call "Mollies", which confuses stuff further, and makes this whole story all the more juicy to me!

Like, the whole thing about salt and Mollies...If you can recall, you're  always told to add "some salt" to their aquariums, in order to keep them healthy, avoiding fungal and parasitic infections. And let's face it- it's fundamentally sound advice for these notoriously touchy fish. The rationale is that at lower (compared to a full-on marine aquarium) specific gravity of say, 1.004-1.015, they're pretty "bulletproof", because most freshwater parasites  and funguses that these fishes often fall victim to just can't make the osmotic "stretch" to this type of environment, and the fish can. Salt in their water gives you a sort of "insurance policy" against some of these nasty problems.  And thus, the old trick of "a teaspoon of salt per gallon" or whatever it was worked.

And, somewhere along the line, the whole "add a teaspoonful of salt per gallon" thing (or whatever the "recipe" was) stuck in our heads. The husbandry recommendation morphed into the narrative: "They are brackish-water fishes. Perfect for brackish tanks!"

True. Well, only partially true, like many hobby things, as I would discover. ("blackwater tanks are dangerously uncontrollable, pH-crash-inevitble dirty time bombs!" - Sound familiar? Yeah.)

And as I became more and more enamored with brackish water aquariums over the past several years, I did more and more research on them, trying to find examples of brackish habitats in which they are native, rather than introduced. I thought to myself, "Hobbyists  always seem to recommend them as candidates for keeping in brackish aquariums- that's the type of habitat where they come from, right?"

And visions of murky, tinted mangrove thickets with mud, Mollies, Archerfishes, and Orange Chromides danced in my head. I just figured I'd find some wild examples from which to draw further inspiration...

But that's where the story gets a bit fuzzy. I mean, these are surprisingly adaptable fish, as long as their needs are met: Warm, hard, alkaline water and low to nonexistent levels of nitrate. And since the toxicity of many of the organics that cause so much stress is less impactful in saltwater, we've used that as sort of "cover" for assuming the types of habitats they hail from in the wild. And of course, being blessed/cursed with euryhaline capabilities, a whole range of assumptions arise about these fish! Throw in the the fact that they come from "down there in coastal regions of Central and South America" or thereabouts creates all sorts of confusion about the types of habitats from which they come!


And it also gives everyone the impression that Mollies are preferentially brackish water fish. I thought that. Absolutely. And, although there are some species collected in brackish water, and even some transplanted populations around the world that live in full-strength seawater, as a whole,  most species have not traditionally been native to most brackish water habitats. Hard and alkaline, yeah- absolutely. But brackish (like 1.004-1.006 specific gravity)? Uh- uh.  They can absolutely adapt and thrive to what we'd consider "brackish" conditions...In fact, I know many hobbyists who've kept them in full-strength (1.021-1.025) saltwater with excellent results, employing them as "algae eaters" in reef tanks and such...or worse- to "cycle" new aquariums.


Now, I'll be the first to admit that I'm not the guy you'd want to turn to for comprehensive knowledge on livebearers, particularly Mollies. I'm just a guy with a more than causal interest in the environments from which they come. This article is not about the best Mollies to keep in tanks, or about the taxonomy of the genus, or any number of other topics that you'd expect from someone who really knows what they're talking about!  My main angle, really, was to see what, if any, species actually come from the types of environments we've been talking about with "Estuary."

Early in my search for suitable brackish water candidates for the "Estuary" project, I looked towards a species I read about which had a very tantalizing common name! 

Poecilia orri, the "Mangrove Molly", which occurs from Central America: Southern Mexico (Quintana Roo) to northern Honduras and Colombia (Islas de Providencia), IS actually found in brackish water habitats, living up to its common name (Don't get me started with the  whole "common name" thing. Remember our piece on the so-called "Amazon Molly?" Yeah...although it is a fascinating fish- ain't from the damn Amazon, much to our collective disappointment here! The "Amazon" part referred to it's unusual "females only" species. ). As we know, common names mean little, really. But this one I somehow recall as being correct. Yet, it wasn't easy for me to verify this, if it weren't for a recollection from MY own travels, as we'll see shortly!

Now, many species of Mollies ARE found in found in brackish "salt marshes" in the Southeastern U.S., along the Gulf of Mexico; there seems to be no easy way of confirming if they naturally occur in the brackish marshes there, or were introduced. Well, there probably is- and I couldn't confirm for myself!

Some time back, while doing my fish-geek due diligence and trying to figure out what type of habitat the various collection localities for the P. orri that were listed on FishBase were, I stumbled on a locale that was familiar to me personally!  One of the listed type localities was "Roatan Island, Honduras, within 0.5 miles of shrimp docks at Oakridge, two sides of peninsula, 1 side clear reef, back side muddy lagoon with mangroves..." Hmm..."shrimp docks at Oakridge..." That rang bell. Yes, of course! I was there on a dive trip some 7 years ago!

While in Roatan, like anywhere I dive,  I asked about spots to dive or snorkel near mangroves and seagrass beds, 'cause who likes looking at stupid sharks and groupers? I remember our guide telling us that there was a "lagoon with a lot of mangroves near the shrimp docks near Oak Ridge..." Eureka! Best part? I remember going there with my friend (also a fish geek), wading out into the sort of murky water, and recall seeing in the brackish water (1.004, I believe- yeah, like a good fish geek, I dive-travelled with a swing arm hydrometer just for that purpose!) what I thought were "wild mollies!"

Yeah, stupid me inadvertently stumbled on a brackish water habitat of Poecilia orri! 

Now, look,  don't get me wrong.

I'm not even going to admit to being slightly well versed on Mollies. I have kept them for years, but the fact that you hear this whole "They do fine in saltwater" thing was floating about the hobby was so generally "accepted" that it simply deserved more investigating! The reality is that there ARE several species which seem to occur mainly in brackish habitats, like Poecilia vandepolli, P. elegans, and the aforementioned Poecilia orri, to name just a fewbut really, they aren't the most common varieties you'll see in the hobby, for sure! The misconception seems to come with the ever-popular P. sphenops and P. know- THE Molly to most of us, along with P. llatipinna. However, P. latipinna does come from brackish habitats as well, further compounding this ridiculousness. Urrgghh!

And, they ARE adaptable to various salinities...even the wild ones. Interestingly, one study I found indicated that isolation in nature of populations P. latipinna in fresh and brackish waters "has not greatly altered their physiological capabilities with respect to ambient salinity."


So...where does this leave us when trying to determine which species work in a brackish water aquarium? Well, for one thing, confused. The reality is that, because of introduction to various habitats around the world, this remarkably adaptable group of fishes has found its way into a variety of niches, and managed to thrive in each! I think- I THINK- that the main misconception in the hobby is that the "domesticated/hybrid" versions that we see in shops are simply assumed to be able to live in brackish systems (which they are, mind you).  However, their ancestors likely originated on hard, alkaline freshwater habitats. We use the salt in their tanks because of the aforementioned husbandry benefits..yet I think we think (LOL) that we're "doing them a favor" and giving them "natural conditions" by doing this (which we may or may not be doing...ughhh). And I'll bet the more hardcore livebearer guys will tell you that, in order to be a bit more authentic, you'd probably want to be playing with the species we've been talking about, among a few others.

This shockingly-less-than-earthshattering, haphazardly-researched "review" of the "Molly Misconception" will NOT land me a keynote at next year's American Livebearer Association convention. And that's okay.  In fact, it's probably setting me up to be schooled by hobbyists who have forgotten more than I'll ever know about this subject! However, in my defense, let me tell you that we in the hobby have, in my opinion, done a remarkably good job at confusing the shit out of the subject of who comes from what habitat. The narrative is just, "Oh yeah, they work in brackish tanks." True, but not all that helpful, really. It's a world of assumptions, inferences, over-generalizations, and partially correct information ( that what they mean by "alternative facts?" Not going there...).

So my comeback to mean/arrogant/knowledgaeble "haters" would be, "You guys did a crappy job disseminating the actual information on this subject!" 


As for me, I think it's fine to include Mollies in your brackish water aquarium, if only for the fact that lots of species of the damn fish are swimming in brackish water habitats around the world! I suppose the "fancy" domesticated varieties would be a bit less "authentic", however. Whether or not they arrived in those locations naturally is a completely different story. So, with a little research, it is relatively easy to find out who is found where. A bit harder to find out if that's the locality where they evolved. Introduction to sites around the world has confused it a bit for outsiders like myself, but it would make a cool topic for some Molly expert to present at some convention some time!

Compelling. Attractive. Interesting. And...weird, actually- Mollies continue to attract fish geeks. They got me, if for no other reason than the confusion surrounding their adaptability. Any hobby topic that has controversy, confusion, and popular misconception is totally "up my alley", as they say! I think they are a sort of "victim" of their own adaptability and variability.

That "euryhaline curse..."

(P. sphenops by Hugo Torres. Used under CC by 2.5 es)

Bottom line: If you want Mollies- any type, really- in your brackish water aquarium and have acclimated them to the environment- go for it. Who the hell am I- or anyone else, for that matter- to tell you otherwise? If you want to be a bit more hardcore, and biotopically accurate, you should consider looking for one of the species we talked about in this piece. 

In the end, this hobby is all about what makes YOU happy. And to me, nothing makes me happier than a good mystery fish! That's the allure of Mollies to me.

Don't always accept the glossy, generalized answer...Stay relentless in your pursuit of the real story. Stay emboldened to try new stuff. Stay excited about the journey.

Stay relentless. Stay engaged. Stay geeky...

And Stay Wet.


Scott Fellman

Tannin Aquatics



Scott Fellman
Scott Fellman


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