i have about 20 tropical fish, what is a good rule of thumb to feed them by.?

Answers:
You don't want any food to be left after about 5 secounds. Real secounds not lightning fast ones. Feed them once a day, if you want to feed them more frequently you could reduce the amount and feed more oftern. If you don't feel this is enough food you could get vacation feeders and always have one in your tank. Live bearers like mollies, guppies, and platies like to eat throughout the day. A vacation feeder doesn't put out enough food to mess with your amonia levels. Remember just because fish think they are hungry doesn't mean they are. Their memory is extreamly short.

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A PIINCH FOR EVERY TWO FISH

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Tropical fish are a self sustaining life form, they don't need to be fed at all.

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I have to agree with animal lover never over feed number 1 rule

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This is a great question.
I would feed what they will eat in 2 to 3 min. If food hits the bottom of the aquarium then cut back on how much you are feeding. You can feed up to 2x a day. It is good to feed a variety of foods depending on the kind of fish you have on what to feed. you can sub this 2nd feeding with a treat their again depending on the type of fish what you would give as a treat( like blood worms, tubifex worms, mysis shrimp, brine shrimp etc) the treats can be freeze dried or frozen (thaw before putting in)
hope this helps you out.