Auto fish feeders that fix onto the tank and have a rotating drum for fish food usually work just fine and donít have a lot to differentiate them. Eheim makes one that is pretty good. Do you want to feed more than once a day, same or different types of food and the hole you have available for the food to drop into are going to be the biggest things impacting your choice. Be aware that if there is a lot of splash or humidity around your tank, feeding flake might now be an option (it clumps up and can get gummed up in the feeder). Nifty features to look for include a battery indicator (sucks to realize your feeder isnít working anymore only after your fish have started eating each other) and an insta feed button so that you can observe fish eat while standing at the tank.
Replacing cartridges depends. Iíd aim to replace them once a month, but you would do better to rinse them out in water you have taken from your fish tank weekly in between changes. If you have medicated the tank, or have something funky going on with water conditions you may want to change more often to get advantage of the activated carbon which is usually doing nothing for you by the end of the month.