i feed my monsters
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hikari bio gold pellets mostly, and they get about 3 twice a day each, my girls get a little less. They are the most complete food out there i find, and they are one of the few fish foods that have an expiry date on them!! (so often fish foods are fed past their shelf life, so alot of the minerals and vitamins are in lower concentrations than recorded on the side of the box, or it's gone rancid due to the fat
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)
I have fed flakes, frozen bloodworms to picky/starving bettas to get them to start eating then switching them to pellets.
Just an interesting note on fish nutrition, i did a term research presentation on it last year and it was actually kind of interesting that there has been very little research into the dietary requirements for aquarium fish. Most nutrition estimates come from extrapolating data from goldfish/trout and salmon feeding trials.
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And many feeding companies keep their research secret
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. So while it may not be perfect it's much better than feeding one particular food. (just think, bettas have bones so need calcium and phosphorus, calcium is very low in invertibrate, so you could get fish with poorly developed bones)