open to suggestions; add a power head or change out the cannister?

My first post here;

I have a 20g long tank that is between being moderatly to heavy planted. (If
things keep growing like they are now, it will definately be a heavy planted
tank)
I am currently using a Finnex PX-360 cannister on the tank and the flow
rate has dropped enough that I feel that the water flow is so low that it is not
good for the tank biology or plant growth.

I have 2 questions to pose to
everyone;
1) should I just add a Koralia 240 power head to the tank to
improve water flow as the tank is clear and the water tests are
excellent?

OR

2) Should I get a CF300 SUNSUN for the tank and use
the Finnex on my QT?

One condition for any cannister consideration is
that it must be able to sit at the same level as the tank as I do not have a way
to place it under the counter that the 20 Long tank sits currently and there is
not another location in my apartment for me to move the tank.

Thanks in
advance for your imput.

Mark

I have posted this over on a couple of other sites. To answer some of the quick obvious questions;
Yes, I have checked the impeller. It is clean and the cannister impeller is in a sealed part of the unit that required a screwdriver to gain access and is on the down stream side of the water flow.

Yes, I changed out the filter floss upon this recent inspection to see why the flow has dropped so fast from 2 days ago. I put new floss in the filter today and the flow is better but there are still dead spots in the tank as a whole.

I chose the Finnex cannister because it is rated to work on tanks from 10 to 30 gallons. I read several reviews on this and the zoomed 511. The finnex had better reviews and was half the cost and twice the cannister tank size, and was designed to work at the tank level. From my research, all other cannisters require them to be placed several feet below the tank in order for the pump to work properly. If anyone has direct knowledge or can give me a link to an article about other cannisters (sunsun cf300 specifically) that will work at the tank level I would love to hear or read about their performance. I rent my apartment and as such cannot drill holes in counters to put cannister plumbing so that is why I need a cannister to work at the tank level.
I do not like the sound or the expense or the sight of HOB filters either on the tank or in it. So please do not suggest I use a HOB filter.

Here is a current picture of the planted tank.