If you feed them once & they don't eat it all in 5 minutes, then you have fed them too much. If you can scoop the excess out of the water do so. If it is in tiny fragments then don't feed them again till the next day round the same time & feed them less. If they eat it all in the 5 minutes then you could probably offer them some more in the same 24 hrs. You will then be able to tell how much & how often to feed them. I have always thought you only feed them once a day. The excess food makes their water dirtier & you will need to clean their tank a lot sooner than if you fed them the right amount. there is no benefit to over feeding them.