You could use a large canister filter with intake and return modified to cope with the wider tank edge or build an overflow (better two overflows in case one blocks up - see DIY section) for a wet/dry system under the tank. I´m not sure whether the fire eel would try to go through the overflow into the sump though as i don´t have that much experience with this species. if that´s the case then you may have to build a cage around the overflow intake.
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