Salt in the Rainbow? Or, the danger of assumptions and inferences...

It never ceases to amaze me how some fishes seem to get lumped into a broad category  in terms of what type of habitat they come from.  

Assumptions, inferences, and downright bad information abounds out there. It's really hard to get accurate information sometimes! This has been really obvious in the more serious research I was doing for brackish water fishes for one of our displays. A friend suggested "Raibows"- and, like any superficially-interested, completely inexperienced non-specialist, I jumped on line and started searching. I was looking for some information to see if there are any Melanotaenia Rainbowfishes that  occur naturally in brackish water habitats. (because I fell into that large group of ignorant, hapless "heathens" that assume every Rainbowfish must be from the classic Melanotaeinia genus, right? I mean, is there any other genus for Rainbows? )

(The beloved M. boesmani...Pic by Eileen Kortright, used under CC BY 2.5)

Once you get beyond stuff like "Jimmy's Tropical Favorite Fish Page!" or whatever, the real hard facts are often more difficult to ferret out. It often boils down to having to sift through various scientific papers and resources, which are often confusing as well. You can also haunt hobby specialty forums, and get really good information from a few people that may comfortably be referred to as "experts."  Notice, I said "a few"- because it seems that even in specialty forums, inferences and assumptions are not uncommon, either...Ideally, you want to contact a person who's actually physically collected the fish in question...not always all that easy! 

It's not just confusing for Rainbows..but for a lot of the fishes commonly labeled as being "brackish water" fishes. 

A classic:

One of my favorite fishes is the "Indian Glassfish", Parambassis ranga. It's one of those euryhaline "switch hitters", in that it's found in both fresh and brackish water habitats. Yet, over the decades, it's been dogmatically characterized as a "brackish fish." The reality is that it's actually more common in pure freshwater habitats, but IS adaptable to brackish water. In fact, if you consult a source like and search for where the referenced specimens were collected, it appears as though every specimen referenced in their database was collected in pure freshwater, typically in areas adjacent to streams and rivers, with no salt content whatsoever.

((Image by opencage used under CC BY- SA 2.5)

Now, the fish CAN live in brackish, but seeing that the highest likelihood is that your fish was collected or bred in pure freshwater, adapting it to salt is not necessary. In fact, many come from water that is typically more soft and acidic! Yet, somehow, most of us immediately think "Brackish fish...add some salt to the water to keep it happy!"


I only include the pic so that you can remember what an atrocity against nature this practice is...

Moving on...

This  confusion/assumption stuff seems to be the case with a lot of Rainbowfishes, actually. For some reason, you'll stumble onto literature that suggests that quite a number of these cool fishes come from brackish habitats. And that is the case with a few species, but you can't make generic assumptions with them! In fact, you'll actually have a fairly difficult time finding truly good detailed information about these amazing fishes outside of some of the speciality groups. 

Even the well-known "Celebes Rainbowfish" is merely tolerant of salt in its water, but it would be hard to call it a true "brackish water fish", occurring predominantly in this type of environment. In fact, it's pretty much the same with most Rainbows. Most species are never found in brackish waters, so keeping any of these fish in an aquarium with a specific gravity at or above SG 1.005 is pushing it, IMHO. But they are hardy and tolerant of slightly brackish water, and will do well in a low-salinity aquarium (like 1.003). 

To most non-specialist fish people, the genus Melanotaenia is like THE genus for Raibowfishes... In popular hobby perception, those are the ones we think of when someone tells you "I keep Rainbowfishes!" Of course, like so many other things in the hobby, it pays to know specialists who really know their stuff...And even then  it's not entirely cut and dry...

Depending upon who you ask  there is a Melanotaenia Rainbowfish that is, according to many hobby sources, supposedly a more-or-less "full-time" brackish-water dweller, the "Black-Banded Rainbowfish", Melanotaenia nigrans.  

It's interesting to note that it's the "type species" of its genus. Although somewhat common in its natural habitat in Australia, it's not hugely popular in the hobby. Yet it seems to be listed consistently as a Rainbowfish that IS found in brackish water. However, when you dig further, particularly into scientific literature, as opposed to hobby literature, it's hard to find a reference to specimens collected in brackish areas, not just "implied" to be in them.  Although it does reference brackish occurrences in the description on, all of the specimens noted were collected from pure freshwater! In fact, the "Fishes of Australia" website of Museums Victoria lists the occurrences for this fish as follows:

"Known only from two isolated populations in NT in coastal streams from Daly River, eastwards across Arnhem Land to Groote Eylandt in the Gulf of Carpentaria, and a Cape York, QLD population known from Prince of Wales Island and a few small tributaries of the Jardine River. 

Occurs in a variety of freshwater habitats including rainforest streams, lily lagoons and small, swampy creeks. Also found in larger streams and rivers in areas of low flow such as backwaters or along the shoreline."

It's not easy to discern from the available literature which, if any, of the Melanotaeinia Rainbowfishes actually are found in brackish water to any extent in nature! Yet, we in the more general areas of the hobby seem to be under the impression that some of them are brackish water fishes. It's kind of like the Mollies, right? Remember our blog on them not too long ago?

I'm totally confused.

Again, that's a classic reason why you join those speciality study groups and get in touch with real experts, many of whom have actually been to the wild habitats of the fishes they love and can give you some pretty good information on them! 

Now, one thing I'm not confused about is that there are some fishes classified as "Rainbowfishes" that definitely come from brackish, even possibly marine habitats: The  Pseudomugil. Yep, Pseudomugil signifer, P.  tenellus, and P. cyanodorsalis will be fine in brackish, and even full-strength saltwater, though somewhat rarely.

(P. signifier. Image by Caliber used under CC BY-SA- 4.0)

Again, from fishbase: "Distribution: New Guinea and adjacent islands, northern and eastern Australia, and parts of eastern Indonesia. Brackish and freshwater, rarely in marine water (2 spp.)"

These are about as cool a bunch of fishes as there is, and they are excellent subjects for our "semi-slaty" world. We'll be devoting some "blog time" to our experiences and thoughts on these species in upcoming editions of  "The Tint" for sure!

And that's where the story ends for now. It's not so much that this is some hard-hitting exposure on the Rainbowfishes and their suitability or occurrence in brackish water habitats...Rather, it's a gentle reminder that we all need to dig a bit deeper before we accept face value on stuff, or make assumptions on information found in our hobby literature and discussions. I've definitely made the mistake of making broad assumptions based on just a bit of information many times, and I don't want to confuse the issues further by perpetuating them. And even then, I know I've personally assumed and shared some shaky stuff's something many of us have, and we owe it to ourselves- to the hobby- to reign it in as much as possible. None of us are perfect. None of us have all of the answers. But that doesn't mean we can't search for them...

Especially when it comes to stuff like Rainbows.

Until next time...

Stay relentless. Stay informed. Stay skeptical, when needed...

And Stay Wet.


Scott Fellman

Tannin Aquatics 





Scott Fellman
Scott Fellman


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