If they aren't eating the feeder fish take them out and put into another tank if you're able, that might also help. Also, be careful about feeding too much feeder fish they can pass on parasites or make your axies sick; esp. if you don't know where they've come from/who bred them prior to getting them from petshop.
Every 2nd day is fine at that size. Once they reach about 20cm + feed them every 3rd day.
If yours eat earthworms, try that more as a staple than the feeder fish (do you hand feed or just drop them in)? Try not to overfeed and remove food if any remains in tank after a half hour.
With gravel being in your tank alot of food/gunk/waste will get trapped between/underneath. River rock/pebbles will also have the same problem. Also, I'm sure you may have read gravel can block them and cause problems later on.
If you are able to remove gravel replace it with sand, no substrate or very large pieces of rock or stone. You could put some of the gravel back in the tank in a kneehigh knotted at top or plastic mesh bag with small holes so gravel won't fall out.
Other alternative is place your axies in a smaller plastic tub and airstone; and change water completely each day. Let your tank cycle (no waterchanges, testing once or twice a week).