For my overloaded aquariums I simply used a vastly over-sized filter system. This prevents me from having to service them too often (throwing water quality into fits), and allows them to digest events like a dead fish that you cannot service in time.
The best filters are the muti-stage canister filters, where they have ceramic media, foam layers, activated charcoal and so on.
I have made my own very effective filter out of a 20-gallon long aquarium and installing dividers to support an overloaded 150 gallon tropical aquarium. Pump in one end, intake from an overflow chamber in the other (it was a marine plexi aquarium). Start with coarse material such as foam or craft stuffing on the intake and work your way to fine stuff such as ceramic media on the pump end. The best part of the aquarium filter is the water level in the "filter" varies, but the main aquarium stays the same. Fill in the bottom aquarium.
I would recommend doubling the size of the filter from what you already think is large if you are filtering cold water. I would suspect, though I cannot confirm, that cold-water slows the filter's effectiveness.
Size the filter according to load, but the fish load has to be consistent to feed the filter so it can do its job properly. Big changes in feeding or fish load will send the aquarium water into water quality swings as the bacteria stabilize for the change.
Other helpers are live plants with bright lighting, and a good PH buffer in the water during water changes. You can purchase PH buffers for marine aquariums such as Calcium Chloride and other materials. Just have to keep tabs on the PH as adding it will effect it. I add the buffer to the change water and test before dumping it in. Also use good aquarium salt to lower fish stress levels.
I am no expert, just roughly 37 years of aquarium malpractice (5yo with first one).
