That is a lot of dust. If you touch a leaf with your finger, will the dust come off into the water?
The immidiate problem that I see is that the filter should be removing those particles but it does not. I suspect the reason being the locolized current from the HOB is not enough to cover the whole tank.
In short, you need stronger water current/circulation.
The easiest way to solve that is to get an adequate filter but that is likely to be the most expensive solution. Something along the lines of Eheim 2213 or 2215 will do the job.
Before buying the new filter I would try the following to see if that would be sufficient:
1. Position the HOB in the middle of the back wall and clean the filter pads.
2. Try finer pads in the HOB.
3. Position the HOB on one of the sidewalls and add a small, cheap powerhead (~$10) on the oposite wall. You want the powerhead pointing towards the intake of HOB.
4. Add a second HOB, with both filters spaced evenly across the back wall. Something like AquaClear 30/50/70. I used 2 AquaClear 30 on a 24 x 12 x 14 low tech tank with AquaSoil. The pricing on these fitlers is strange: 70 can be cheaper then 30. They have adjustable flow and getting a larger filter then you think you need is never a mistake.
Personally, I would skip all of the above and just get a decent canister filter. It would clear all that dust off the plants in a day and keep them clean.
Bump: looking at your tank some more, especially the first picture, the AquaSoil is now mostly mud. You might as well just replace it. You can use, say, a 1/2" ID hose to suck it out.
Last edited by OVT; 09-19-2018 at 12:45 PM.