When was the last time filters were cleaned? What are your dosing, light, and w/c regiments? What does "lots of pressurized co2" mean?
I have a HOB filter with two sponges. I replace just one at a time, whenever I feel like it, probably every 2-3 mths on average.
It's a 10 gal with 40 W CF light, bulb was changed a year ago. Dosing NO3 to ~15 ppm, PO4 to ~1.5 ppm, 1/4 tsp KCl, plus traces as per directions. Sometimes a bit of iron. I admit I'm bad at doing the mid-week dosing, so the nutrient levels do get low, it's my bad. I also have way too many fish, so there's a ton of fish waste. Fish have always been healthy, not a single illness or unexpected death since I've been keeping these fish, over 10 yrs. Lots of plants, including some fast growing ones. Pressurized CO2 is 2 bps, with a sprial ceramic diffuser. I don't have a drop checker in there anymore, but I know that this rate results in a "green" range (and I did use proper 4 dKH water that I made up myself). I just lent my drop checker to someone, that's why it's not in there anymore. Plus, it collects algae and looks ugly.
hope this helps