A good quality complete nutritional supplement or grain free 'ration balancer', or even better, do a diet analysis so you can ensure you're supplementing the horse with precisely what it needs. There are specific hoof-related supps, & cheaper supposedly 'complete' ones, but in my experience they often lack necessary ingredients or quantities, so analyse them carefully. Single ingreds, such as biotin(not often too lacking anyway), magnesium, seaweed, etc only give one out of many nutritnets that could be lacking/imbalanced. As the hooves are just epidermis, feeding the whole horse properly will ensure the hooves are well fed too.
Diet, as opposed to just nutrition, can influence the feet and grainy, sugary feed or even too much rich grass, or too much feed & a too heavy horse can negatively effect the feet. Feedxl.com & safergrass.org are good resources.
Cracks often(usually?) start at the ground surface and are caused by bad mechanics - long hoof walls, flares, incorrect balance, etc. No amount of correct nutrition is likely to fix these unless the mechanics are taken care of also. If you think this may be a problem, or if you're not sure, you can post some hoof pics, as some of us here may be able to give you a better idea about the trouble.