Hi and WELCOME!
These are 'general rules' and are best to be followed based on your priorities. Having to buy corals to feed your fish is costly and irresponsible IMHO and what will likely happen if you try and break these 'general rules'. There are caveats, however. Sometimes you can get non-reef safe fish trained to only eat flake, pellet, frozen, etc. but this generally requires a time commitment and a fish only set up for a number of years. Then you can place them in your reef. This is still a gamble though. A trained fish may be fine in your reef for any particular period of time then one day happens to take a nibble on a coral or invert and, well, that is the end of that saga. I have done it with a few fish and invariably all "trained" fish will still take some nibbles. Best to keep them well fed with quality food-stuffs and hope they don't acquire a taste for your favorite coral.
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These are 'general rules' and are best to be followed based on your priorities. Having to buy corals to feed your fish is costly and irresponsible IMHO and what will likely happen if you try and break these 'general rules'. There are caveats, however. Sometimes you can get non-reef safe fish trained to only eat flake, pellet, frozen, etc. but this generally requires a time commitment and a fish only set up for a number of years. Then you can place them in your reef. This is still a gamble though. A trained fish may be fine in your reef for any particular period of time then one day happens to take a nibble on a coral or invert and, well, that is the end of that saga. I have done it with a few fish and invariably all "trained" fish will still take some nibbles. Best to keep them well fed with quality food-stuffs and hope they don't acquire a taste for your favorite coral.
Hit Like and Share often!