Just sounds like a matter of needing finer mechanical filtration to me. I've used the Aquaclear sponges (bought the largest size for the 500 model and cut to fit various Marineland Penguins) or the generic foam inserts and filter floss from Big Al's online.
Though be aware if you add fine filter sponges/floss, you'll have to keep on top of squeezing them out more often so the filter doesn't completely clog up.
I would purchase a Fluval 306 canister for that tank & be done with most filtration problems within days. You could add some carbon as well for a short time & the tank would clear right up. Remove the carbon in a week or so until clear . The new filters from Fluval are good. I would not run HOB filters on most planted tanks unless breeding shrimp in smaller tanks.
You could leave 1 of the HOB filters if you are in love with them. They make good Quarantine tank filters but poor performers at planted tanks. They expel too much CO2 from the tank. Dual canisters or 1 HOB filter with a strong canister combo.
Before buying another filter, I would try one of two things. First, try putting fine filter floss in. Put it as your second to last layer as you want something to hold it in place. The other quick and easy way would be to get Fluval prefilter sponges, assuming they fit on your HOB intakes. If they are Aquaclear filters from an AC 10 to an AC 50, they should fit perfect as that is what they were designed for. They will fit many other filters as well.
Note that the media will clog fast, either way you go. Not a problem as you want to get all the fine stuff out. I had the same problem with 2 AC's before and never thought much about it, added shrimp, thus adding the prefilters, water cleared up a lot.
Use the filter catridges for the HOB, tear off the blue floss, empty out the crap carbon, and use them as gates, and stuff them with non-toxic pillow stuffing from any crafts store. I've had good success putting pantyhose on the intake too for cloudy dusty water from new sand in new tanks.