1. for your rena's intake tube, how low does it get to the bottom?
2. how big is your tank?
3. how many filters are you running?
4. if only 1, if 55g or larger I suggest adding an AC unit (110 or 70), gives you more filtering power & allows you to alternate cleaning.
5. but DIY the AC unit's intake tube with pvc so it reaches the bottom & go a step further by making the intake into a bottom tee so you have to inlets.
In my case for my 72g BF, I run 2 AC110's, 1 unit with the double tubes given so it reaches mid level, the other unit I DIY as stated above.
So I have coverage & with the great water flow both provide (if you don't have too much plants/ornaments/rocks to diminish/impede the power of the flow), my left mid level unit pushes the heavier poop that doesn't float that settled in the center of the tank towards the right sided tee, while the other right side DIY unit sucks it up. + I have a sponge filter.
My poop problem is solved (then again I have bare bottom so it's easier for the poop to move along the bottom) & when I do w/c I will vac at same time, pretty efficient set up for me.
so your expensive rena doesn't suck, just needs some cheap labor to assist in the cleaning!
check out
www.kensfish.com for great selections/prices/low shipping costs.
all I am suggesting is larger tanks require 2 filters as even 1 very strong filter (rated for much larger tanks) can't possibly move all that heavy poop to get to the one intake tube!
for the price of 1 rena xp3 could have gotten 1 AC110 + 1 AC70!
good luck!