Outside of feeding true piscivores, feeder fish if fed are a treat/dessert, perhaps a couple of feeders per week.
Please, do not feed Goldfish or Rosy Reds, they are high in fats* and thiaminase**, neither of which are good for your predatory fish.
Feed common tropicals, e.g. Guppies, Platies, Tetras and etc.
These are inexpensive and not usually kept in the same abhorrent conditions as the more common feeders.
These still need to be thoroughly quarantined*** and treated proactively for common diseases/parasites.
You can also raise your own feeder fish.
Many tropical livebearers are very prolific and if your willing to maintain the extra tanks then culled fry/juveniles make excellent feeders.
Raising your own feeders is a lot of work, but if you already have a healthy well established Guppy tank or a pair of Convicts then maintaining a growout tank is fairly easy.
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PRO
Feeder fish when properly quarantined/raised and conditioned prior to feeding are a healthy, nutrition packed meal.
Disregard the oft espoused nonsense that feeder fish have little or no nutritional value.
Some keepers think that the feeding of feeder fish is good for the instinct/psyche of the predatory fish, that exercising their natural instincts makes them healthier/happier fish.
In some ways I'm inclined to agree...
The larger the habitat, the more closely the habitat/water chemistry mimic natural ideal conditions and the better their dietary needs are met, the fish often grows faster/larger, display better coloration/patterning, more inclined to breed and so on.
Let's face it, many of us enjoy watching our predatory fish eat feeder fish/live foods and there is nothing wrong with'it as long as the foods are not fed purely for a sadistic pleasure.
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CON
Why feed feeder fish at all?
Your fish may not want to chase feeders, can't blame'im, why work when pellets come from the hand of God.
Your fish may initially try and give up, happens alot, then you have feeders living with your O.
Your fish could injure itself trying to chase feeders.
Your fish could die from a disease or parasite.
Your fish may decide to eat nothing but feeders, then you'd have to start all over again breaking'im to pellets.
Why all the bother, just to see'em eat another fish?
Why all the bother for something you might ideally feed once a week, one or two feeders... Or even less.
Let the feeders stay at the LFS and die en masse there.
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I have and very likely will again feed feeder fish to my predatory fish, but my mainstay for live foods are and always will be, nightcrawlers, crickets and mealworms.
I recommend alternative live foods that pose much less risk of illness/infestation, e.g. crayfish, shrimp, worms of all types, crickets, meal-worms and etc.
Nice thing is that other than washing or gut loading these live foods no other precautions are really necessary.
Also, these are usually easier to obtain, transport and maintain.
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*
Fats
Cold-water fish are higher in fats and a steady diet of these can result in 'Fatty Liver Disease'.
MFK article said:
Although not a actual disease, fatty liver is a serious condition in which the liver becomes enlarged, often to the point that it interferes with, or even crushes, the other internal organs and is apparently the cause of death. This condition seems most commonly to be the result of poor diet, and the consensus of several well-known fish pathologists is that the single most common cause of fatty liver disease is a diet high in saturated fats, although biotin and/or choline deficiencies, toxemia and "unknown nonspecific causes" are also possible factors.
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**
Thiaminase
http://www.ansci.cornell.edu/plants/tox ... inase.html
Thiaminases are enzymes found in a few plants and the raw flesh and viscera of certain fish and shellfish. When ingested these enzymes split thiamin (Vitamin B1), an important compound in energy metabolism, and render it inactive.
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***
Quarantine
A good quarantine consist of observation and proactive treatment lasting at least two weeks, prefer a month or even longer.
1)Observe prospective feeders at the LFS very closely, appearance, swimming, respiration, feeding and etc...
Any dead/unhealthy fish in the tank or adjacent tank(s), don't buy any.
2)Before introducing to QT, give'em a strong salt dip/bath, this will force many external parasites to drop off the host.
3)QT should be kept at 87'-88'F and salted at at least 1tblspn per 5gal, this will eradicate any ICH and some parasites.
4)Feed at least one week with antibacterial food and then at least one week with anti-parasite/protozoan food.
5)During this time observe closely for anything indicative of illness/infestation, treat accordingly with meds or just cull.
A proper QT is alot of work.
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