Need filter suggestion for cichlid tank

I have a fairly newly setup cichlid tank, and thinking that my filtration is
marginal. The tank is 125G nominal capacity but actual water volume is more like
100 gallons. Size is 6' long, 18" wide, and 20" high. The population consists of
six 3.5" to 5.5" Lake Malawi cichlids, mostly peacocks, no mbuna. Also six 3.5" to
4.5" clown loaches and a 6-7" common plecostomus. There is an assortment of plants
around the sides and back. Substrate is 3-5mm smooth quartz gravel. Large pieces
of slate are stacked on the right and left sides two high with maybe 2" gaps. (The
clown loaches favor the bottom story on the left.)

Filtration is a single Fluval 404 drawing from a suction tube at the right side
and low. The return is split three ways: two Marineland Pro30 near the middle of
the tank and the Fluval return nozzle at the left side. The nozzle gets most of
the flow, maybe 50% of the total 340GPH. This filter is rated for 100 gallons
maximum, and at this rate the filter could turn over the tank around 3 times per
hour. I assume that the biowheels add significant extra biological capacity.
Marineland rates them for 30 gallons each.

I don't think there is really enough flow in the tank to effectively draw debris
from one side of the tank to the other and into the filter inlet. Especially with
the rockwork, I don't know if the filter is going to create any meaningful flow
under and between the slates. I recently took apart much of the rockwork to do a
thorough gravel vacuuming and found several pockets of junk. It was a lot of work
and I wouldn't want to have to do this for every gravel vacuuming. My thinking is
to add additional circulation or filtration, but am unsure how the fish would like
that or how effective it might be. The tank is too close to the wall to use a
hang-on overflow or behind the tank filter of any type.

Any comments or suggestions ?

George