I doubt the filter(s) can't cope with the stocking.
It's a Jewel Rio 180 (180 litres) stocked with 3 swordtails, 1 rtbs, 1 bosemani rainbowfish, 1 'disc tetra', 1 angelfish and 3 corydoras plus I'm running a Fluval 204 external as well as the standard Jewel internal.
If anything, it's...