Why aren't fresh water feeder fish ok?

[Michael Hoaster]

For years and years I've heard that fresh water feeder fish are not adequate food for salt water fish. Is this based on science? Or is it based on assumption? Can anyone provide scientific documentation one way or the other?

I ask because it seems to me, with salt water fishes' osmoregulation, their flesh is no saltier than fresh water fish. So their flesh should be no different. If salt is not the issue, what is? What are the differences that make fresh water feeders inadequate?

And what about mollies? These are brackish to full strength seawater fish. They eat salt water algae, even dinos and cyanobacteria, which I have personally witnessed. These would seem to be ideal, being prolific live bearers, effectively turning algae into feeder fish.

Is this a long-standing myth that needs to be debunked?

 

[MarAquatic]

I feed my lionfish fresh water guppies so I dont know

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[ThRoewer]

I think that it doesn't have anything to do with nutrients but rather the introduction of certain internal diseases (Ichthyosporidium, fish tuberculosis,...)
If you breed your own feeder fish you should be good. If you have to buy the feeder fish I would be careful.

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[nereefpat]

Fat content and makeup is different. Goldfish also contain a substance (thiaminase) that binds vitamins, causing deficiency.

 

[Michael Hoaster]

I've been looking around the web for answers. Turns out, this question has been asked before - a lot.

So far I've found that fresh water fish have much more fat in their tissues, which can lead to fatty liver disease. Also, as nereefpat posted above, they have thiaminase, which breaks down thiamine (B1). So your fish could end up with a B1 deficiency.

I'll keep looking and hopefully will get more responses here, but the gist of what I'm seeing is that fresh water feeders are like fatty junk food to salt water fish. So it's ok to feed them but not as the only food in their diet.

Goldfish seem to be the worst choice, and brackish fish like mollies are better but not as good as salt water fish.

Can fresh water fish diseases survive in salt water and infect salt water fish?