Yes, a symbiotic relationship, but NOT FOR YOU AND YOUR FISH.
It is a symbiotic relationship between your fish's death, and the money from LFS feeders and the LFS itself.
Though not all LFS consciously sell the deadly combo on purpose (mostly cause they are merely following other LFS examples), i do see a cunning design for profit.
They sell pacu and oscars, usually without any info other than "they are aggressive". They don't tell you to quarantine feeders, rather they silently want you to just feed them to your fish straight off the bat.
they take the chance that most people will put these fish in smaller tanks, and/or keep poisoning their fish with feeders from LFS. all the while raking in money. Fish make big diarrhea = more money when amature fish keepers replace filters as they stop off to buy even more feeders to poison their fish with. the fish will die alot quicker (if not from an undersized tank), and more pacus and oscars will be purchased with the amature fish keeper ignorantly thinking "wow, i had that pacu for a good 3 years... fish must not live very long"