Thanks all - it's definitely not the filtration, the flow is well distributed across the compartments, and it's only cloudy in that photo as the water had just been changed moments before.
Only now I've noticed you'd mentioned PhosGuard. Removing phosphates is rather dangerous play. You see no results at first except algae disappearing. So you're happy but then you realize that your plants have ceased to grow. And then, without apparent reason, they gradually begin to die off. Those with tubers or any sort of nutrient stock hold better, longer, but in the end, they'll die, too. Phosphorus deficiency is often an invisible killer.Something seems to have changed, I have no idea what.
Only now I've noticed you'd mentioned PhosGuard. Removing phosphates is rather dangerous play. You see no results at first except algae disappearing. So you're happy but then you realize that your plants have ceased to grow. And then, without apparent reason, they gradually begin to die off. Those with tubers or any sort of nutrient stock hold better, longer, but in the end, they'll die, too. Phosphorus deficiency is often an invisible killer.
Does my description fit your situation?