Hi all,

I recently collected five 1.5-2.5cm Red River Pupfish from a tributary of the Cimarron River, which is pretty darn salty. I went ahead and tried them in fresh water, and they were (are) doing great for a couple weeks, or so I thought. They were active, eating well, and most have put on some size. Yesterday, I realized that 2 were gone. Looking around, I only found the remains of one. The remaining three were actively feeding, chasing each other around, etc. last night. One is even starting to show the flash colors. This morning, another one is dead with abdominal cavity completely hollowed out but fins all intact. They share the tank with 3 Fundulus kansae, 5 Gambusia, and a single small Notoropis atherinoides. Some ramshorn snails ate the dead one, I'm sure...

So any ideas as to what could be knocking them out? Is the salt-to-fresh water change finally catching up to them osmotically or something? I've never really messed around with salt in aquariums, or switching a species back and forth from fresh to salt, etc., but I know they are found in fresh water too... and though the plains killies do chase stuff around in there, I haven't seen any ragged fins on anyone.

Although it's true that they live in fresh water, I have always found them to be much more hardy with some salt. They always seem to fade away in my main tank, even though our well water is practically liquid rock. Also keep in mind that pupfish are omnivores that need quite a bit of vegetable matter in their diet.

Wow, home from work and a 4th one is dead. This was literally a fish I saw alive and greedily eating about 4.5 hours ago at lunch, developing the purple-blue colors and having grown substantially since capture. The body is immaculate, with a nice full belly, perfect fins, and clear eyes. It's difficult for me to understand how such an active little fish in such great condition could deteriorate to death in such a short period of time, unless it's a salt chemistry thing I'm unfamiliar with. They do get some vegetable matter for food as well...

Well, I guess I'm going to have to break down and buy the salt and hydrometer and try a new group because I've been absolutely smitten with these little things thus far. Thanks for the input.

Edited by shoegazer, 04 February 2009 - 06:42 PM.

Obviously, your pups are infected with tiny little alien chestbursters. You must have caught them right after the facehugger stage.

I'd have to guess that the salinity drop caught up to all of them at about the same time, as would be expected, but whether that's what got them or if they were nailed by the other fish once they got weak I could only guess. I have the same problem with common Sheepshead Pupfish; without salt they just don't last a month.

I can't tell you what happened to your fish, nor have I kept that species before.
I do have considerable experience acclimating several brackish species to freshwater. A partial list includes mummichugs, rainwater killies, sailfin mollies and both three and four spine sticklebacks. In some cases these were collected in full strength ocean water. I acclimate them to full strength fresh tap water without any additional salt. I have had great success with excellent longevity and multiple generations of captive bred offspring.
I do have hard water which is pH adjusted with KOH to above 9.0 by the town water department. This may work in my favor allowing me to leave out the additional salt.

My method is simply to take a piece of small diameter plastic tubing [maybe half or a third the diameter of airline tubing] and get a syphon going from the freshwater tank the fish are going to into the collection bucket [a typical five gallon bucket with only a few inches of water]. Once a syphon is going, I tie a knot in the end of the tubing and snug it up until the water is dripping at no more than 3 or 4 drops per minute [estimated]. My goal is to keep the drip going for 24 hours minimum without overflowing the bucket. If needed I will pour off some of the water to keep the bucket from overflowing but the 24 hour minimum seems to be important. Care should be taken to be sure the temps stay the same between the tank and bucket which is difficult going from a heated tank to a bucket sitting on the floor.
I have used the same system to go from freshwater back to full strength saltwater as well.

Having said all that, I will note that some species do better than others when changed from salt to fresh. Mummies for example seem more sensitive and sometimes seem prone to fungus in the freshwater. Rainwater killies on the other hand can virtually be taken from ocean water and dropped right into freshwater without apparent harm.
It's possible C. rubrofluviatilis is more sensitve. Try again using the drip method, maybe even keeping the drip going longer than 24 hours.

Thanks for the input, guys. I did in fact, slowly acclimate them to pure freshwater over the course of 10-12 hours and didn't just drop them straight into freshwater but perhaps as you mention, that was not sufficiently long enough. I don't think this stretch of water could be more saline than 10-15 ppt. I went ahead and added some salt last night and I'll see if the last one hangs around longer.

I would definitely check the salinity of the source water. I don't have a clue what conditions are like in your area but if the water is very highly saline, acclimation may be more complicated than I described.
I would also consider the possibility that some other factor is at work here. My gut feeling is it may be aggression from another tank mate.

I'm headed out tomorrow to pick up a few more fish and I'm bringing a hydrometer. I have to think it was the total lack of salt in my tank that decimated them. I haven't lost this other one, clearly a female full of eggs at this point, after adding salt to only ~1ppt. As I dropped the salt in, this fish literally swam over to the area where I poured the salt and just rested in the little hazy saline waves for about 15 minutes as the salt dissolved. I'm thinking I wont have any further problems.

Yeah, the salinity where I caught the original animals was off the chart of my cheap little brackish hydrometer...more than 15ppt, and probably 22+ so I actually managed to catch some further upstream at about 1ppt.

If it were me I'd suspect the pups of killing one another, or perhaps the fundulus.(saw/suspected similar with a bunch of flagfish juveniles in which bodies started turning up in this manner) But the clean fins doesn't fit this story

I'm thinking it HAD to be the total lack of salt at this point. After adding even a tiny bit of salt (~1ppt), the last pup from the previous batch is still alive and well. All the new pups are fine too.