congrats on getting poisoned
hahaha no issues with internal filtration. IOUS or IOS also can. if you worried about fries or sand getting sucked in, then block off the underflow with sponge also can and leave it.
fries, if you decide to let them be and not harvest them out, most of the time stays near parents for a start, so if your shells and rockpiles are away from the inlets, they should be fine.
so your filtration if you have the internal plus an OHF should be good enough. but if you wana add a cannister, why not? haha
main thing is make sure you cycle your tank well before stocking up.