The advantage of a bigger pump section is that evaporation will not cause as much cavitation like it would in a smaller section. Baffle 4 and the pump are one volume so if you make the pump smaller, enlarge the other.
The space for water level fluctuations should only be for power outages or else the incoming water will wash over the 3 media sections.
The other issue is when the fine filter becomes clogged, water may back up in the prior section and wash over baffle 3.
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