Good quality guppy food means food that contains highly digestible protein. The most digestible proteins are those that come from shrimp, fish and other marine animal species. Food with a lot of wheat or vegetable filler is not as nutritious and will pass through the guppy undigested. It will foul the tank. Some live foods are more nutritious then others. BBS (baby brine shrimp or artemia) is the best starter food for guppy fry, although it should be fed to parents more sparingly. Generally guppies will do really well on a high quality dry food and BBS. Some guppy breeders have also reported success feeding decapsulated dehydrated brine shrimp eggs or golden pearls instead of live bbs. The advantage of feeding a high quality food is that you can feed less, resulting in less guppy waste and less maintenance. So high quality food is cost effective.