Well I am finally sick of paying for feeder fish to supplement my Oscars diets, and running the risk of introducing medical problems. Now what I am doing is converting my 10g tank into a tank to grow a few plants out in and to store a breeding colony of livebearers that will be used to feed the Oscars. My question is what live bearer will reproduce quickly, but attain a decent size? Guppies are out because it would take too many of them for one feeding decimating the breeding population, so which of the others would give me the best results?

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75g Blackwater Oscar tank, 55g rescued Koi tank, 55g Native tank, 55g Community tank, 29g Community tank, 25g Bolivian Ram tank, 20H GBR tank, 20L Black Convict, 20L in progress, 10g Fry tank, 10g Dragonscale betta tank, 3g Crowntail betta tank.

Mollys if you want a decent size, but most livebearers (including Mollys) take quite a while to get to full size, some a year or more.

Your best bet would be gambusia or even guppies as you mentioned, because although they're small, they breed and grow the fastest of all the livebearers so you'll end up with hundreds in no time

I have also had a recommendation for Convicts before, but I may take a second look at the guppies.

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75g Blackwater Oscar tank, 55g rescued Koi tank, 55g Native tank, 55g Community tank, 29g Community tank, 25g Bolivian Ram tank, 20H GBR tank, 20L Black Convict, 20L in progress, 10g Fry tank, 10g Dragonscale betta tank, 3g Crowntail betta tank.

Mollys will start breeding at about 5 months,but they won't grow large enough fast enough in a 10 gal, convicts would work but not in a 10 gal. You would need at least a 30 gal for a pair.

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Thanks Susan, I was thinking about a 40 breeder if I went with convicts because petco is about to do another dollar per gallon sale here. The 10g would be used if I could find any smaller fish to do the project with.

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75g Blackwater Oscar tank, 55g rescued Koi tank, 55g Native tank, 55g Community tank, 29g Community tank, 25g Bolivian Ram tank, 20H GBR tank, 20L Black Convict, 20L in progress, 10g Fry tank, 10g Dragonscale betta tank, 3g Crowntail betta tank.

Convicts have weapons - spiky pectoral and dorsal fins that can hurt a predator's mouth. If they are big enough to be a meal, they are big enough to spike a predator.
Technically, if they spike inside the throat (the Cory suicide defence) it could be a problem, but I have seen convicts living and breeding in piranha tanks because the piranhas want nothing to do with them.
All cichlids have that feature... and a brood of convicts young enough for their spines to be soft would be one meal for an oscar.
I had a huge number of Herotilapi multispinosa at one point - hundreds of one inch fish that I couldn't give away. I ended up offering them as feeders, but nothing would eat them because of the spining.
Gambusia are also tough. They are fast growing, but they are also ferociously cannibalistic, so are lousy feeders. It would take about 20 20 gallon tanks to generate enough guppies, mollies, swords or other livebearers to feed an oscar regularly.

so basically I may want to look into a shrimp colony?

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75g Blackwater Oscar tank, 55g rescued Koi tank, 55g Native tank, 55g Community tank, 29g Community tank, 25g Bolivian Ram tank, 20H GBR tank, 20L Black Convict, 20L in progress, 10g Fry tank, 10g Dragonscale betta tank, 3g Crowntail betta tank.