If you want to feed your cats and dogs natural food then you need to feed them BARF (bones and raw feed) the stuff you buy in a tin/pouches isn't their natural diet, it's just processed crap even if it is specially formulated to meet your pets needs... e.g. Koi food contains mostly wheat germ, which is a really good growth food for them but not exactly natural. It helps them grow quicker when mixed with other nutrients, it might not be natural but it is a good food source for them. As long as your fish get a varied diet, it looks healthy then I don't see the problem with some supplements. It will still be getting some marine algae from it growing on rocks etc. As long as any food added is removed before it starts to rot it shouldn't pollute the tank.